I have 3 nexus 6 and all of the phones had the speaker lowering issue I tried swapping speakers batteries and even the boards to no luck all phones we're running car droid my issue was I was missing alarms and sleeping in after giving up and normally updating to (S.i.X.AOSP.ROM-R3Ds [N6-WORKING-BT][09-24-17]) I was surprised to find my volume issue was FIXED my phone never reduces volume ever no matter what the battery percentage is so it was a software problem all along and not hardware I proceeded to install on my 2 other phones and I can conform all 3 phone audio issue is fixed maybe it's the kernal used in this ROM I have no idea what fixed it but I'm happy and not changing Roma anytime soon so to the people with audio issues try flashing S.i.X.AOSP.ROM-R3Ds [N6-WORKING-BT][09-24-17] might fix ur issue