From https://docs.microsoft.com/en-us/windows/win32/api/mmeapi/nf-mmeapi-mixeropen
1) Can't use the WAVE_MAPPER constant:
uMxId
Identifier of the mixer device to open. Use a valid device identifier or any HMIXEROBJ (see the mixerGetID function for a description of mixer object handles).
A "mapper" for audio mixer devices does not currently exist, so a mixer device identifier of -1 is not valid.
2) Can't use the old workaround of opening the device first with waveOutOpen and then using the handle:
"On 64-bit systems, this function may not work as expected in situations where you pass a 64-bit LPHWAVEOUT pointer in the uMxId parameter, because the uMxId parameter is truncated to 32 bits."
Are we just supposed to accept that there's no way to control the mixer?