There's a note in the troubleshooting part of the Boxer release notes about the CD audio issue... unfortunately the answer is that at the moment, DOS games aren't able to play their CD audio tracks in Boxer.
However, this lack of CD audio support is a limitation I have been meaning to follow up on here, as I may be overlooking something obvious about DOSBox's CD audio handling on OS X/Linux.
[boring developer stuff]
The problem is that OS X represents CD-ROMs that have both data and audio as two separate volumes - one for the data, and one for the audio. DOSBox only sees the data volume, and doesn't 'connect it' with the corresponding audio volume, so the DOS emulation doesn't see any audio tracks on the CD.
This seems to happen regardless of the use of the -ioctl, -noioctl or -usecd mount options. It's a different story on Windows, where the filesystem represents a data+audio CD as a single volume rather than two.
[/boring developer stuff]
All that said, there is one way you can get CD audio to work: you can make a BIN+CUE image of the game's CD-ROM, and place the image files into your game package. Boxer will mount them automatically, and DOSBox will correctly read the CD audio information from the image.
Unfortunately I don't know of any free OS X utilities for making images in this format - the only ones I know of are on Windows. Roxio Toast will make BIN+CUE images, but it costs $80 and has no trial version. And it doesn't appear that other disc image formats are able to store the data+audio in a way that DOSBox can use.