Solo guitar and piano music can also be decent choices provided you avoid songs with extremes of high and low volumes and pitch. Most phone systems come with default tracks that, coincidentally, fall squarely into these categories. If you’d like something a bit different then you can find plenty royalty-free tunes available to purchase online (they’re not all that terrible), but remember you'll need to pay for a PRS license if you want to make use of tracks by a recording artist.
If multiple files are uploaded, they are played in the order that they are listed on the Music On Hold page. If there is only one file, it is played repeatedly.

A customer on hold represents a great opportunity to advertise services and products. If your system isn’t, you’re missing out on the one in five callers who make a purchase based on commercials heard on hold. We offer comprehensive phone message ordering options, including:
“At Page, Smith & Douglas, we pride ourselves on both our extensive knowledge of tax law and our dedication to superior customer service. We understand that tax problems can be intimidating, so we have a completely open door policy for our clients to come meet with their attorney in our office or contact them anytime by phone or email…”