Hi,
I want to know if there are open source audio post processing like bass management, THD, surround sound, loudness control, virtual surround etc are available somewhere ?

thanks,

Are you talking about real time processing, where you apply the affects during playback?

Usually, post processing means processing that's done in the studio after recording (after the talent has left).

Well , it does not matter .. Usually it is after we get the audio from decoder.. which is pcm ..

Most post processing is done with an audio editor such as Audacity (open source), GoldWave, Adobe Audition, Pro Tools, etc. Frequently, optional plug-ins are used. The surround-sound stuff is pretty specialized.

Real time processing is most-often done in hardware, although some soundcards come with utilities that can apply effects (including surround sound processing). I don't know if there are any open source tools to do this, since (I think) it's usually done at the driver-level, and is closely tied to the particular soundcard.