If you change your sound device while RuneScape is running, RuneScape will not pick up on the new sound device and you need to restart the RuneScape client to get sound back.
I noticed this, you may have to disable all other sound devices. I switched my audio from headphones / line-in to monitor speakers and although youtube did play audio through the monitor, I noticed RS sound was still playing through my headphones.
Bob says:
A bank PIN will keep your items secure.
Always check the second trade screen.
Never trade in the Wilderness!
Keep your computer keylogger-free and virus scanned.