Why does the Sonos app not have a 0ms delay option for the line-in adapter?

The delay is part of the way Sonos works. It's not the line in adapter. It's present in ALL Sonos devices. There is always going to be a delay on the line-in, it needs to buffer the input in order to stream it to other Sonos speakers in your home.

The only speakers where this is not present is in soundbars and the ARC input on the Amp to allow the audio and video to be in sync. When you group a speaker to the room. the second and subsequent rooms will have a delay.

If you haven’t seen a post about this delay, just do a search in this community. It comes up dozens of time a week, in various threads. 

https://en.community.sonos.com/speakers-229128/line-in-delay-6887947#:~:text=The%20delay%20is%20part%20of,lip%20synced%20with%20the%20video.

I understand why there are delay options available for the reasons stated above, however I still feel there should be an option for 0ms (or as close to that physically possible) should a user wish to make use of that feature.
It seems silly not to have that option for scenarios where one doesn’t need a delay.

Is it that difficult to implement?

I can’t speak for Sonos on this, but I can give my opinion. While there may be many who wish to incorporate analog systems into the Sonos ecosystem, Sonos is mor concerned about whole home audio. This requires speakers to speak to each other to achieve a seamless audio experience. Would it be simple to implement a setting that eliminates the line in delay? I believe it is possible if it can be done for the Amp and soundbars. Will this implementation be a wise business decision? 
I believe it would be because it could/will open a whole new market for Sonos, like DJs adding it for wireless audio at gigs.

the bigger question though is.. is Sonos ready for that  in light of the recent changes that is still causing issues for many?

For me that would be a rather large unneeded expense as I’d have had to pay for four adapters that I’ll never use.

For others that need the Ethernet option it would be even more aggravation as they would have to pay for both adapters.

So what happens when people flood the Sonos Support line because they set their line-in delay to 0 and their multi-room music system starts stuttering and dropping out due to an insufficient buffer?

It’s only possible for the soundbars/Amp due to the low latency, one-way, ad-hoc 5 GHz network made especially for surrounds and/or subs.  That connection is not robust enough to go through walls and floors with any reliability, hence the use of 2.4 GHz and a buffer for anything meant to be grouped between rooms.  Which is why the delay is still there when grouping a soundbar/Amp with another room for TV sources,

As to opening their market up to DJ’s, the only thing separating Sonos from a standalone PA system is multi-room streaming.  The OP wants the 0 length buffer for “scenarios where one doesn’t need a delay”.  So they want to use it without the multi-room function, i.e. a standalone PA system (which is what they should buy in the first place).   Horses for courses. 

A random few milliseconds of delay when browsing a web page or setting a thermostat is no big deal, but it would be a major disruption to audio play. SONOS players are time aligned within about two milliseconds. The 75ms latency gives the system some time to work around network delays while maintaining the time alignment. Sharing multiple time aligned audio streams with up to 32 players is not so easy. Eventually, competing products licensed SONOS patents, else their product was limited to servicing a very limited number of players and time alignment was spotty.

If you have two identical CD players, each playing a copy of the same disc, simultaneously start playing the two discs. By the end of the first track there will be an obvious time misalignment between the players. On the other hand a SONOS system can play all day and into the night while maintaining time alignment (within two milliseconds) between multiple players.

I’m not aware of any 3rd party alternatives to the line-in adapter.  Last time I checked there are plenty of options for USB C to 3.5mm adapters available on the market, but no 3.5mm to USB C adapters.  These adapters are not 2-way.

Theoretically, Sonos would have to go into a ‘no grouping’ mode, where the line in source can only be played on the connected room. I say theoretically as I think there would still be  stuttering issues with a speaker pair/ sub in the room as I believe those speakers are connected via 2.4 wifi and do need the buffer as well.   But even then, I don’t think people would be happy with this. 

Sonos could maybe do what they do with TV audio, play immediately in the connected room, and delayed elsewhere (still runs into issues with speaker pairs).  People don’t like that either.

The purpose of the line in was not so that users could connect it to TV/vdieo sources, or to integrate and sync with other audio systems, it was to bring in external audio sources like turntables, CD players, etc.

This is not even factoring in that wifi audio because substantially les reliable in a crowded room to the additional interference.  There have been horror stories posted here of people testing their wedding reception venue in an empty space successfully, then getting dropouts during the actual event

Indeed.

In my opinion, providing a 0ms delay option in the app would be a practical and user-friendly feature, especially for those who primarily use a single speaker, such as the Era 300. Any potential confusion or technical issues when using multiple speakers could easily be addressed with a simple pop-up explainer in the app. This guide could offer personalized recommendations and best practices based on the user's specific speaker configuration.

Introducing this feature would significantly enhance the utility of the speakers, enabling a wider range of creative use cases, all while staying true to their core purpose and functionality.

In essence, adding a 0ms delay option doesn’t need to undermine the multi-room functionality or overburden Sonos Support. It simply provides users with more flexibility to tailor their Sonos experience to their needs. Empowering users in this way aligns with Sonos’s premium branding and innovative spirit, while addressing a growing demand for low-latency solutions.

I’d love to hear Sonos’s perspective on the feasibility of such a feature and whether it could be explored in future updates.

I’d like to add another perspective that I think resonates with a significant number of Sonos users, myself included.

For many of us, the appeal of Sonos lies in its ability to enhance and complement an existing audio setup rather than completely replace it. Not everyone has the means—or the desire—to overhaul their entire analog system or speaker collection built over years of enjoyment. Often, integrating a single Sonos speaker into an established setup can provide incredible value, such as richer sound or enhanced bass, as is the case with my Era 300.

In these scenarios, a low-latency option for the line-in adapter would allow users to enjoy seamless integration with their existing equipment without the frustration of synchronization issues. This isn’t about competing with Sonos’s multi-room ecosystem; it’s about recognizing that many customers don’t use (or need) a full Sonos system to reap the benefits of its premium technology.

I’d also wager that there are many users out there in similar situations—where Sonos speakers are used to complement, rather than replace, their beloved analog or legacy setups. For these users, offering a 0ms delay option would be a thoughtful and practical addition. It would allow Sonos to cater to a broader audience while maintaining its core strengths.

Sonos has always been about flexibility and innovation, and providing a feature like this would align perfectly with that ethos.

Perhaps you just purchased the wrong product to meet your needs, ​@Rickardo? Are you within the return window?