StevenShortly I know this may sound funny but have you tried Total Commander? It is an exceptionally good file manager and it comes with a built-in audio player (plays video too) which is like a very no frills folder based music player but it does the job for me and I love it to bits.
https://play.google.com/store/apps/details?id=com.ghisler.android.TotalCommander
I installed Better Batter Stats (BBS) to figure out my drain. Turned out to be a low quality signal where I put my phone at night.
These permissions are needed if you install BBS:
adb -d shell pm grant com.asksven.betterbatterystats android.permission.BATTERY_STATS
adb -d shell pm grant com.asksven.betterbatterystats android.permission.DUMP
adb -d shell pm grant com.asksven.betterbatterystats android.permission.PACKAGE_USAGE_STATS
adb -d shell settings put global hidden_api_policy 1