My BT volume level on the phone is always 100%. When I first start Spotify, I hear a song start playing at normal volume, for maybe 1/2 a second, or 1/10 of a second. It immediately drops to a very low volume (although the volume still shows as 100%). The volume continues at this level as long as the device is connected. I've had this experience with multiple BT devices (car stereo, JBL speaker, my garage stereo system that has a BT input, etc). It has ONLY been an issue with Spotify - Tidal, YouTube, etc work fine.
Discussion
How do you have these settings in Spotify configured have you tried experimenting with them?
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Normalize - off, volume level greyed out but Normal is selected.
Try the settings I've shown, as I've never experienced it, I have turned it off for now to see if it affects anything.
No luck. Setting it to loud did give a little increase, still at 50% or less volume.
Are you on free or premium?
This is definitely an app/service issue and not related to the OS, plenty evidence of the issue being reported across the web from Android to iOS and multiple device variants and bluetooth accessories. If this was OS related it would impact other apps too.
I would tend to agree, likely something in the way the app uses the OS audio drivers. I had an identical Pixel 6 that worked fine, bought this one to put Graphene on fresh, and it has never worked. Everything else has been great, though. I appreciate you talking me into Graphene, or at least pointing me at resources that were convincing.
Premium family, locked in hard enough I don't want to also pay for Tidal.