3CX started as a Windows-based software PBX and now runs on Linux and in hosted form. A default install ships one music-on-hold file, onhold.wav, in the instance’s prompts folder. On Windows that’s C:\ProgramData\3CX\Instance1\Data\Ivr\Prompts\onhold.wav.
We couldn’t find a composer, title or license for the stock track, so there’s nothing we can play here. Admins replace it under Settings → Music on Hold.