On average, 5 to 15 seconds of music plays between each message blurb. Keep your audience in mind: musical breaks that are too long are boring, but message blurbs that are long or too detailed can be overwhelming. In most cases, 5 to 15 seconds, between message blurbs that don’t exceed 30 seconds are usually appropriate.
It makes sense to keep these callers informed while they wait with Custom On Hold Messages… informative messages, mixed with music.

“Call center operators who picked up the phone reported feeling less emotionally exhausted when dealing with customers who heard standard pop music.” –Time, “Why Music on Hold Drives You Crazy”
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.