Ideally you would have to make them yourself as the third rule for the jam is:

• Assets must be made during the jam — exceptions are logo/intro/fonts

This is somewhat similar to LudumDare rules for Compo submissions and although it's restrictive there are a bunch of ways to go about it:

• Create them yourself, even if they are crappy or very simplistic
• Use procedural generation tools to create them
• Team up with someone that can make them for you
• You can create derivative work from a pre-existant asset (by modifying them enough they can be distinguishable from each other)
• Don't use audio (find a reason to not need it, or make a game that can do without it, or leave it for later... this may result in a low Audio score though)

HOWEVER! If non of this works you may opt out of the Audio category by mentioning that the audio was not made by you or your teammates on the 72hs of the jam.

This will result in a much lower Overall result since people won't vote on the Audio category.