I think these should be further split, both to aid reviewng (each of the four drivers' implementations needs checking against the new one) and to help with tracking down regressions. Additionally it would be nice to have better commit msgs rather than 'n/4'.
I'd suggest moving the functions added in `IAudioClient3` first; then `IAudioClient2`; then the rest, one function at a time, with perhaps `Start`, `Stop`, `Reset` going together. Perhaps some of the more simple ones can be merged too.