That’s not normal.

How close are the Era 300s to your listening position?

Can you even hear any height or surround audio when the sliders are at the minimum setting?

I don't have Apple music but Amazon sounds really good on my Ultra, Sub  and 100 Room and on another Room with a 300 Pair and a Sub.

I have very little faith in test tracks, the few I have tried didn't seem to correspond to what I was hearing from streaming or TV sources.

What is your listening configuration and room information?

Sound as you probably know is very subjective. Yours is the first post I’ve seen about the “Surrounds AND Height” volume levels being too loud. 

• My question is….did you have a baseline (meaning another similar surround system) for comparison to arrive at your conclusion regarding the sound levels?
• If you have an iOS device, did you try TruePlay tuning?

Regardless of the above it appears you tweaked the levels to your liking for the most part. That again falls back to a subjective listening choice.

Calling Sonos Tech Support may help. However, in situations like yours regarding sound they typically rely on several registered complaints about the same issue before initiating a deep dive into possibly tweaking the sound characteristics of a speaker.

There’s one difference I would have made in your configuration and that is the choice of Sonos Sub. I realize that cost may have been a deciding factor. However, with Arc Ultra and Era 300 as surrounds a full-size sub is the recommended product. The excerpts below can be found here in the FAQs.

What are the recommended products to pair with Sub Mini?

• For the best experience, pair Sub Mini with Beam*, Ray, Era 100, One*, One SL, or an IKEA SYMFONISK speaker. Sub Mini is also recommended for smaller bookshelf speakers powered by Amp.
• *Any generation will pair with Sub Mini.

Will Sub Mini pair with Arc?

• Yes. Sub Mini works best with Arc in a smaller room and at low-moderate volume levels. If you have a larger space or intend to listen at higher volume levels, Sub will provide a better experience.

Of course, all the above is JMHO. Good luck to get your system sounding as you like.

My baseline is what's coming from the Front L & R drivers. When I say "too loud" I mean that audio coming from the height and surround channels is too loud in that it totally overpowers what's coming from the front and centre channels. 

Music mixed in Atmos sounds all wrong. The ambient background vocals from the height speakers set to 0 volume shouldn't be overpowering everything else to the point I can barely even hear the main tracks from the front & centre drivers is what I mean.

I still don't really like the sound. I've tweaked it something more acceptable by turning the satellite speaker volume sliders to the minimum possible and turning off Trueplay & setting surround distance to "less than 2 ft" even though they're 9 ft away from me at least. The rear surrounds (not front as I wrote above) are still too loud but I can't turn them down any further. 

I got the Sub Mini because anything larger would be too overpowering for me and the space I'm in. The Sub Mini is almost too much as is. That choice is a subjective determination though. 

They're about 3.5 m away from me. I've also turned off Trueplay and set speaker distance to "less than 2ft (0.6m)" which turns down the rear surround volume a bit more. Still too loud though.

I can definitely hear plenty of sound from heights & surrounds with sliders at minimum.

It's only an issue when listening to Atmos mixes from Apple Music embedded in the Sonos app. Multichannel TV and Blu-ray is fine. I wonder how many people actually use Apple Music in the Sonos app for Atmos music anyway. Maybe not many and maybe that's where the problem is. 

Play 20 minutes of Apple Atmos and then submit a diagnostic. Call Sonos to discuss your issue.