Reddit reviews Modern School for Xylophone, Marimba, Vibraphone (Morris Goldenberg Classics)

Yes, definitely the Goldenburg Book.

And what will also help is just looking at parts on imslp.org. It's practically history's sheet music at your finger tips. Just print out flute or violin parts, just any C instruments for that matter. Have fun and good luck!

Any of the Morris Goldenberg 2-mallet etudes would be good. For 2-mallet stuff, something short and sweet is best, but play musically.

Link to Goldenberg book: https://www.amazon.com/Xylophone-Marimba-Vibraphone-Goldenberg-Classics/dp/075790890X

For 4-mallets, it really kind of depends on where you are technically. Yellow After the Rain, Rain Dance, and Frogs are all do-able and would be good audition pieces if you've played some 4-mallet stuff before. A solo out of the Mark Ford Technique through Music book would also be good. Find a piece that will show off what you can do technically. Don't reach for a piece that you can just barely play through. If you're struggling through a piece, they'll know. That being said, the piece should show off a wide variety of techniques, so a chorale isn't really a good audition piece. But, a more technical piece with a chorale section is great.

I like all of the recommendations for Timpani pieces, too. I'd just add that the Vic Firth etudes have some good choices, and the Beck Sonata is a standard in timpani rep, so doing mvmt 2 or 3 out of there would be cool, too. Good luck with your auditions! Where are you looking to study?