We're not talking jingles this time. If you've ever seen (and not heard) a multimedia advertisement devoid of music, you probably noticed that something was eerily absent. You may have also realized what it was.
A commercial without music in the background is a lot like a sitcom without a laugh track:
It's jarring.
That stark silence can be useful. More than a few PSA's or ultra-serious topics benefit from not having any sound other than the stern dialogue about drugs or abuse or forest fire prevention.
But that's the exception that proofs the rule. Generally, music sets the emotional tone for the entire experience that the viewer (listener) has during the very short period that ad has to garner their attention and cater to their wants and/or needs.
Upbeat music for alcohol is common. Dramatic music for action figures and comforting music for dolls is, as well. These are cliche but for good reason, they work.
Wanna know what has become cliche and stopped working? Ukelele solos. Please stop using them, it's been a decade...somebody do a bass cello or a keyboard. Heck, I'd take a lute or steel drums at this point.
But my big recommendation: If you have a commercial that has a turning point, don't be afraid to show contrast with two different songs. Even a thirty second ad, if it features a stark turning point ten or fifteen seconds in, can make use of this rare and effective technique.
Bottom line: don't neglect the background stuff, it makes as much a difference in your presentation as the dialogue and imagery.