All I can tell you is that I've had it work for me reliably, three times in a row with the same Sim, who didn't have fertility treatment and didn't even own a television, I don't know whether she had the Family Orientated trait though, I suspect she might have and I can check later. It was also her favorite music, I don't know if that makes a difference. Also I literally had her listen to it every waking minute for the whole pregnancy from 'try for baby' onwards.
My experience with multiple births in Sims 3 leads me to suspect that the way multiple births is decided is something like this
<average or total of parents fertility> + <random number> = <initial fertility for specific pregnancy>
with that initial fertility determining an initial number of potential births something along the lines of: below a specific threshold zero births, ie no pregnancy, above that but below another threshold one potential birth, between that and another threshold twins, above that triplets.
Actions taken during pregnancy such as watching kids tv etc then push that number up and if it later crosses one of the thresholds you get an extra birth (or two).
I have no proof for this, but it's my gut feeling about how it works. I'll try and see if I can come up with some kind of test for it.