HELP - Sound output fluctuating

[TomHG92]

I've been having this problem for a while now in that the output sound from my iMac, say when I'm listening to iTunes or using the music software Reason, randomly changes, getting quieter for a while then bursting back up quite significantly. It has become quite frustrating now, as I'll increase the volume to better listen, when suddenly the sound output can come back up to normal level and be to high. I'm really unsure to what is happening, is this due to CPU usage or some other technical fault of which I have no understanding? Please help.