Too many subjective terms there. The Arc is going to play (normally in stereo, not 5.1, depending on your stream) and at the levels you set, both by the original streaming company, and your setting of the volume controls. And in stereo means fewer of the internal speakers will be involved, since there is nothing on the extra ‘channels’. If you can (have a streaming source that supports it), I’d also suggest you try an Atmos stream, it may be that the additional speakers involved will help.

Finally, if you’re using surrounds with your Arc, I’d suggest going in to the room settings for the Arc, and set the surrounds to ‘full’ rather than ‘ambient’ for music playback. 

Thanks Bruce

Im Streaming using Spotify via the Sonos app. I still need to try airplay from Spotify without going through through Sonos app to see if that makes a difference. 

its just confusing because all I have done is swap the beam for the arc. Same tv, same Location etc

I didn’t have any of these issues with the beam, I get it’s a different speaker. 
 

the sound is amazing via the tv/hdmi, streaming via the app and the sound is nowhere near as good. 

Then I would recommend that you call Sonos Support directly to discuss it.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Your Spotify steaming experience should not be worse with the Arc Ultra vs the Beam. Do you have another streaming service you can try? If you don’t then I suggest you setup Apple Music on a trial basis  as you can cancel before it goes subscription. The only catch is that you can’t get trial status if you had Apple Music and canceled less than a year ago. Of course there’s also Amazon music.

I don’t know what genre of music you prefer however the Arc Ultra may handle it differently. Another point (and this too depends upon the genre) is that the Beam projected sounds in more of a straight line; for lack of a better description, whereas the Arc Ultra produces a wider sound stage. 

Of course most of what I presented regarding sound is speculative. I suggest you run a diagnostic within 10 minutes of playing music and submit it to Sonos (make note of the reference ID). Don’t post the reference ID in this forum. Just call Sonos Tech Support. The diagnostic may not show anything wrong with the Arc Ultra; but it’s still worth considering.

Finally, have you tried any of the tuning options in the app?

EDIT: Looks like that guy ​@Airgetlam stole my thunder again suggesting a diagnostic. I’m gonna have to break his fingers