The audio will be modulated and sent along with the video to the hackrf, so it still only needs one. This is how it works for 625 now, I just need to add an AM audio option for 405. SDR allows any kind of signal or signals to be transmitted / received, as long as they all fit within the bandwidth of the radio. The HackRF is ~20MHz, so there is plenty of room. In theory it could transmit several channels at the same time, but I've no plans to do that with hacktv