Picking a solo song can be an overwhelming process. It's hard to find a song that can highlight all the skills that you'd like to show. Sometimes, you may have to be a little strategic. Additionally, if you get offered any callbacks, you will likely have to choose a contrasting song to your audition.
Speaking personally, I (Claire, the creator of this website and a member of the DJs), chose July by Noah Cyrus as my audition song. I chose this because I knew my range check earlier in the audition would take care of showing my higher range, and I wanted to further present my lower range. I also wanted to show my mix. Additionally, it's just a song that I really enjoy singing and listening to. For my callback song, I sang Million Reasons by Lady Gaga. I chose this because, while it further explored my lower range, it was more of a ballad than the slightly more upbeat July, and allowed me to belt within my belting range, a skill I didn't really show during my audition.
It's almost impossible to get a song that will perfectly be everything you want it to be, so focus on a few main skills that you want to present, and see what comes to mind. Remember that you can change the key of a song (sing it a little lower or higher than the original singer), so don't let that stop you from picking a certain song. Just know the note you'd like to start on, as groups will play that note on the piano before you start.
If you're really at a loss for what song to choose, just think about the songs you're most comfortable singing. Stage presence is part of this, so the more comfortable you are, the more you can emote and show your energy to the groups. Hopefully, this can be the "fun part" of your audition where you're just showing what you can do!
Note: Groups may specify that your solo song cannot be musical theater or classical