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.
When was the last time you were placed on-hold and felt like this guy because you had to listen to: -A message recorded (probably by an employee) and added to, over and over again at various levels?

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When you work with a professional audio production company, that uses legally licensed music and professional voice over talent and has experience editing audio for telephony applications, the sound quality is going to be much better than something put together by an amateur or something you've recorded and thrown together yourself. Professional audio production is very cost-effective, and you'll get a big bang for your buck by going professional.