mciSendString() setaudio volume error 261

I'm using MCI to do some sound-related stuff, and everything works, except I cannot alter the volume. I have the following code:

mciSendStringA("open res/theme.wav type waveaudio alias maintheme", nullptr, 0, nullptr);
MCIERROR error = mciSendStringA("setaudio maintheme volume to 50", nullptr, 0, nullptr);

error is 261. The program works fine but the volume does not change. Any suggestions on what's wrong? (Two pages of google searching and there's nothing)