For those still using analogue or digital phone systems, voice on ISDN circuits has a bandwidth of around 4000Hz. The audio from an analogue phone is sampled at 8000Hz, and because of the nyquist theorem, must be low-pass filtered to 4000Hz. So any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are good at recognising human speech, so this is considered "toll-quality" voice, i.e. good enough to pay for. This is the reason why certain vocalised letters are difficult or impossible to tell apart on the phone. For example, the differences between B, P, T, C, etc. are almost entirely above 4000Hz.
The included USB drive is preloaded with 6 hours of royalty-free music recorded specifically for music on hold with standard pleasant "thank you for holding" messages inserted every minute.

"Messages on hold are often one of the most painful of customer ordeals. But when done right by Scott Greggory and crew with actual humor, it takes a customer hassle and elevates it to a customer experience you tell others about. Click through to check out a couple samples. They're addictive, and brilliant marketing." - Bill Cusick / Author & Customer Experience Expert
There aren’t many things people hate more than being put on hold when they call your company. They expect to talk to a person and to be treated like one, and instead, they’re ignored and have to spend time waiting for someone to answer. It’s annoying, and it can cost you business. Yet, there are simple ways to keep people on the line if they’re on hold, and the best one is to write an on hold message script.