my girlfriend is a beastie boys fan, and she says that they play all the instruments that you hear on the albums. not the most credible source, but i'll take it.
for what it's worth.
No, she's correct. They play their instruments for not only their instrumental jams and punk songs, but they actually play the instruments on the samples they loop into their songs. While they still use samples from a handful of outside sources, a large chunk of what they mix are snippets from a massive library of jam sessions they've had over the years.
Kind of funny since they used hundreds of different artist samples in Paul's Boutique. Didn't someone calculate approximately how much it'd cost to make that same album today? Its outrageous.