I’m not sure what it covers. You would need to take it to Apple if any stores are open. Or Contact Support. There’s returns that can be mailed in or you can use authorized dealers to get it fixed. Without the Apple Care I think there’s the standard one year warranty. I’m not really sure so just call Apple. You have three months of free telephone support Call 800-MY-APPLE ... Mind you everything is very busy or slow due to the pandemic.
If it was me I’d want a full refund or exchange.
It shouldn’t lose Bass this soon into having it. We usually see these reports once it’s over a year old. I’ve had mine a few years and have not lost bass.
Good luck! Do try other music though.