It helps to specify which version of the engine you’re on. Audio mixer is still experimental and we’ve not quite tested all cases yet! I think the issue might be that the VOIP stream is using a USoundWave vs a USoundWaveProcedural and the audio mixer is casting to a USoundWaveProcedural in the lower level code. Somebody else ran into this.
I think if you change the async task ptr in FMixerSource::ReadMorePCMRTData for bProcedural case to USoundWave vs USoundWaveProcedural and avoid the cast, it should work.
In any case, this should definitely be fixed since we recently used VOIP in audio mixer for our last SIGGRAPH demo.