ao_oss with no -softvol

4Front OSS and FreeBSD OSS (but not DragonFly) support per-application volume. volume_oss4() already implements ability to control it within mpv.

4Front behavior:  the last app closing /dev/dsp sets default volume
FreeBSD behavior: default volume is set to hw.snd.vpc_0db (45 by default)
                  and depends on hw.snd.vpc_autoreset (1 by default)

sysctl() bloat can be removed if you think handling non-default configuration is unnecessary.