It may not work in dosbox, but mushroom.ovl is just raw 8-bit[1] linear audio sampled at 8kHz so you can load it up in your raw-audio-capable app of choice. On linux try "aplay mushroom.ovl", for example.
[1] Actually only 6 bits per byte are used; presumably this was done to improve decoding speed or something.
The result is 6 bits of resolution, along with a high pitched squeak during playback, depending on the output circuitry and speaker characteristics (it's less noticeable when a low-pass filter is added).
[1] Actually only 6 bits per byte are used; presumably this was done to improve decoding speed or something.