I’ve found a workaround, so I’m just writing this for anyone else who runs into a similar issue. After I upgraded directly from Ubuntu Mate 16.04 to 17.04, I started checking out the upgrade (love it, many thanks to all the developers) and found one issue. When watching video using MPV, the volume is very low. Interestingly, MPV volume now maxes out at 130% instead of 100% and my system volume maxes out at 152% instead of 150%. In any case, with everything maxed, volume was at a whisper, but worked fine using VLC for comparison. I resolved the issue by creating a config file for MPV: ~/.config/mpv/mpv.conf and added the following entry:
ao=alsa
Audio now works as expected. I assume this is some sort of driver/hardware/upgraded MPV issue, so if it’s useful to anyone, my audio hardware shows as:
00:1f.3 Audio device: Intel Corporation Device 9d71 (rev 21)
Subsystem: Hewlett-Packard Company Device 819f
Flags: bus master, fast devsel, latency 32, IRQ 134
Memory at b4328000 (64-bit, non-prefetchable) [size=16K]
Memory at b4310000 (64-bit, non-prefetchable) [size=64K]
Capabilities: [50] Power Management version 3
Capabilities: [60] MSI: Enable+ Count=1/1 Maskable- 64bit+
Kernel driver in use: snd_hda_intel