Haiku developer François Revol has done an initial port of the Open Sound System to Haiku. OSS has been open sourced by 4front Technologies recently, and powers the audio hardware of many Unix derivates.
OSS features an impressive list of supported devices - the port aims to seemlessly integrate with Haiku's media subsystem, the OSS drivers will just work like any other of the currently available audio drivers.
For further information, please refer to this announcement made on the OSS developer mailing list.