Hi @Kdesilva
Welcome to the Sonos Community! And, apologies for the delay.
Sorry to hear of the issues you’ve had trying to stream movies from an offsite Plex library, but I do not think that Sonos is responsible.
Every DTS-HD or DTS:X soundtrack contains within it a standard DTS core - the DTS core is lossy, and the other formats pad this out with extra data to increase the quality and to perhaps add more channels. Plex knows this, and when presented with a DTS-HD or DTS:X soundtrack and a renderer (the Arc) that only supports DTS, will extract the DTS core from the bigger stream and hand only that core to the Arc. None of this will affect the incoming stream and therefore cause buffering - I think your friend of a friend is assuming that his Plex server is needing to transcode the soundtrack to DTS in real-time as it supplies the movie to your system - this is not the case (at least in this instance, with these codecs).
Note, however, that DTS-HD and DTS:X are not formats that you will find a streaming service use - they are typically only found on Blu-Ray due to the excessive bitrates they employ. So, if Netflix (just for example) considers DTS-HD too big (DTS:X is even bigger!) to stream with the fat data-pipes they likely have available, I think someone’s home internet connection is going to struggle - especially if a second user is watching another movie from the same Plex server at the same time.
In the Plex client, when looking at a particular movie but before playing, there is a 3-dot menu - go into this and select Playback Settings, then Audio track and if available, choose a Dolby Digital soundtrack - this will reduce the bandwidth needed to play the movie. It may also help to check the internet connection to whatever device your Plex client is running on - use 1Gbps (or better) ethernet if possible.
I hope this helps.
