The right background music feels entertaining for the caller. This makes the time until they are connected with an employee seem to pass more quickly.
You can support this perception very well if you use speech in between in your on-hold message. An example of such a message: "Please be patient a little longer. The next free employee will be with you shortly". This way, callers feel a little less lost and are usually more patient.
Passing on information about the remaining time is also a good idea. For example, the position in the current queue or the approximate remaining wait time. This way, callers can better estimate how long it will take.
This approach is especially recommended for long waiting times. In this way, you give your customers the chance to perhaps try again at a later time.