Do the following to enable or change your Hold Music:
1. Click the cog or gear near the top of the Skype for Business client.
2. Select Ringtones and Sounds. Note that the option Play on Hold Music may be greyed out.
3. Click Browse then simply select an audio file on your PC. Note that the audio file must be in .wma 16bit 8khz mono format.
4. You are done!