[Sylwester]

You can't just catch it, because it's handled client-side. I needed this before and I searched a lot, but I found nothing that could easily solve this problem.

Since server is transmitting data to clients, then it must be possible to detect it, but it probably requires messing with memory.

I found a way to detect when player press +voicerecord button (though it's not very good and clients can mess up results if they know how):
client cvar "voice_modenable" is always set to 1 when client press +voicerecord, so you can constantly query it and set it to 0 whenever query returns 1 (client just pressed +voicerecord), but I have no idea how to detect when client stops using it.