So I know that some schools use slideroom for musical supplements. For schools that don’t, do I just upload them through the portal? Or is the portal only for supporting documents and new accomplishments etc. I have my recordings, I just don’t know where to submit them. Thanks for any help.
Brown uses Slideroom for music supplements. If other schools you’re interested in don’t, there’s usually a place on your applicant portal to upload items–but from my experience you cannot upload recordings unless you put a Google Drive link in a document----so I wouldn’t recommend that. You might try contacting the Admissions Office and asking—that’s why they’re there! 
Yes, contact admissions! You should also consider submitting a music resume and letter(s) or recommendation from teacher or director related to music.
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