Let’s drop some jargon: Caller retention. Customer satisfaction. Reducing churn. If you care about any of these, hold music should matter to you. Music affects mood. It can even have an effect on purchasing and decision-making behavior. Skip the sponsored Instagram posts of avocado toast; hold music is a bonafide influencer!
It's a panoptic experience, being on hold while listening to someone singing about being on hold. Depending how long it takes for someone else to join the call, you could be listening for awhile. The music builds as time goes on, culminating in a spoken verse that begins: "Well, let me tell you all a story / about a man who was on hold all day..." This unusually self-aware song is the default music for a conference call service called UberConference, where "I'm On Hold" plays on a loop for anyone who calls in early, on almost every call that doesn't start exactly when it's scheduled to (or doesn't ever start at all). It's selected by nine out of ten call organizers, and plays over a million times per month, says Craig Walker, CEO of Dialpad, owner of UberConference.

You also need to factor in your average wait time. If you know your callers will be listening for a while, choose something other than rock or alternative, which creates the perception of longer wait times.
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.