DSOAL and OpenAL Soft are free software under the GNU LGPL (see the licence files here).
ROTK Fix ships them unmodified, in ROTKFix\sound (dsound.dll = DSOAL, dsoal-aldrv.dll = OpenAL Soft).

Source code for the exact versions shipped:
  DSOAL        https://github.com/kcat/dsoal          commit e11128250439d586483ee6598d1670ab52ab5e37 (r697)
  OpenAL Soft  https://github.com/kcat/openal-soft    commit 9ca7d05e4f1b16897104c913be7553891f0d027d (r10817)
Binaries: https://github.com/kcat/dsoal/releases/tag/latest-master (DSOAL_r697.zip)

The source of both, at exactly those commits, is included in the Athelas download: licenses\source\
dsoal-r697-e111282.zip and openal-soft-r10817-9ca7d05.zip.

You may replace these two files with your own builds of the same libraries.
