Hello Rutto,
Bpod includes a convenience function for generating chords of sine waveforms: GenerateSineChord().
The output of GenerateSineChord would then be loaded to the HiFi module with H.load(). It could then be played from the PC with H.play() or from the state machine with the 'P' command in the OutputActions section of any state definition.
Unfortunately the HiFi module does not support playing multiple loaded waveforms simultaneously, so you can't load sine waves as separate sounds and trigger them to play concurrently. Any new sound triggered will replace the ongoing playback.
-Josh
