I followed all the steps in the software, but I still get the M&G/Fun With Pets loading music/screen every time. I haven't hadded custom music and splash screens yet. I also don't have a loading screen replacement.
My settings look like this: https://imgur.com/a/mPug420

Do you have any suggestions what I could adjust?

Note: I've tried to contact the creator through Tumblr, but they haven't responded, hence my question here.

I've had the same problem. I followed all the steps in the software and double checked my settings but the music just wouldn't randomize. Then I tried with version 1 and that works for me.

@Corax thanks this also worked for me. However, did it also work for you to add custom music?

I'm happy to hear it also worked for you. I never wanted custom music so I haven't tried adding any, sorry.