Is there an audio filter/effect that works like the Broadcast Safe effect but rather than setting video within legal limits it assures sound is within legal limits (-18)?
You'd want to run the audio through a compressor which will ensure that nothing peaks. That is the only thing technically illegal for broadcast. I believe soundtrack has one.
I do a lot of broadcast stuff for CBS, and they want the audio to never go past -10, so you can compress it to -10, but it is better to bounce the audio out, lowering it on a submixer or even just the master mix, using -10 or -20 tone as reference.
Odds are, if you just keep it from peaking, where-ever you are going to broadcast is going to run it through their own hardware compresser/limiter anyway, they will also have to run it through a video "setup" which rises your blacks to 7.5% IRE for NTSC broadcast.
The goal is to make sure that your tone sounds comfortable to your ears at its given level, and everything else is sounds good, relative to the tone, without peaking.