The same goes for music and models too. You don't get penalized for using free assets, we just need you to disclose it (and trust me, we'll recognise most of the free ones if you don't ;3 )
Importantly, you *must* not use any music that will flag DMCA. Because all of our entries will be streamed, we are very strict on this part. We don't want an 8hr judging stream demonetized because of this.
There are many excellent free-use audio libraries out there, or you can hop on an asset store and buy some licensable tracks.