Bluetooth songs are playing in lower sound

honestly i personally would check to see if it's the bluetooth headphones themselves, make sure they don't have their own volume slider and make sure the volume your phone has set for those headphones are right because iPhones "remember" what volume a bluetooth headset was at when it was last paired and by default it should be like, idk, around half way when freshly paired? maybe less, or maybe the iPhone will set the volume of that headset by default when first paired to the volume of another bluetooth device, it can happen sometimes, so basically with just the information you've said, i'd suggest making sure the bluetooth device is set up properly, the iPhone's volume for it is set right (for my bluetooth headphones that'll be around half way, maybe a little less), and also maybe try re-pairing the headphones with the phone, also make sure its not just your bluetooth headphones, if this doesn't help feel free to ask more and i or someone else can help further, there are a couple other possibilities it could be but these are the main things i think it is, but again, if these don't help feel free to ask more, and if possible also clarify a little bit more as well on if this has always happened and also i'd like clarification, if this doesn't work, on if this is system wide or if its only with apple music, i hope this helps and as always, feel free to ask more questions and i or someone else would love to help