Unfortunately, Synergy does not currently support the distribution of audio from one computer to another.
One workaround is to use an optical audio cable (S/PDIF or TOSLINK), by connecting the output on one computer to the input of the other. Then enable "Listen to this device" in sound settings (which mixes in the audio from the other computer) and use one computer's sound as the primary output. This would give you a cleaner, digital signal, which is less likely to suffer from interference.
If this article didn't help, you can talk to someone on our support team (unless we sent you here, in which case please reply via the ticket you already have open).