If you had earbuds cranked-up loudly, I agree it could drown-out a siren - but I bet an onboard stereo blaring loudly on top of wind-noise would be just as likely to if not more so.
Don't know that to be a fact, but I do know that with regular earplugs (and a full-face vs. half-helmet) you'll generally hear outside sounds *better* since plugs filter out so much of the wind-noise. I bet it's the same deal w/ buds (as long as it's not cranked way up).
It would fun to roll around rocking ABBA or something totally ridiculous & unlikely just to see all the expressions though!
edit: In fairness, my perspective is pretty skewed towards quick riding out in the country on a sportbike, not cruising behind a full-fairing where I'm sure onboard tunes work better & make more sense. One of these days....
As far as earbuds being breaking the law -
that would be the least of my law-enforcement worries if/when I eventually hit the trooper jackpot at a more.... spirited point in my typical rides.