I've been suffering this exact same problem. You'd think there would be a legitimate fix by now.
Anyway, I came up with a slightly more elegant solution than running Rhythmbox all the time, since it uses more memory and cpu than I'd care to give up on my tiny netbook. Plus, I don't like the tray icon being there all the time.
First I created a one-second silent .ogg file in Audacity. To do so, simply start Audacity, click on Generate > Silence..., enter 000,001 seconds, and export it as an .ogg file. I saved mine as silence.ogg and moved it to /usr/share/sounds/.
ogg123 is a command-line ogg-player that should come with every system. It uses a very minimal amount of cpu and ram, so it's perfect for running in the background and playing, well, silence. The easiest way I've found to implement this is to create a .desktop file and add it to /etc/xdg/autostart/, like so:
This will run every time you log in, so you shouldn't have to worry about it ever again. Plus, it doesn't interfere with any other music you may want to play.
Comment=Prevent wireless from shutting off when inactive
Exec=ogg123 -r -q /usr/share/sounds/silence.ogg
Hope this helps somebody.
If anyone has a better way of doing this, let me know.