PC-SFX

 PC-SFX is a real-time digital audio signal processor. It
 makes use of the CPU processor to add digital effects to the
 audio signals fed into the sound card. These effects are
 added on-the-fly (full-duplex operation).

 PC-SFX is capable of performing Chorus, Digital Delay,
 Reverberation, Distortion, Pitch Shifting, Equalization, Noise
 Gating, and Volume control. See section 4 (Effects) for more
 details. These effects can be performed with 8-bit resolution at
 44.1kHz or 22kHz. On a 486 SX33, all the effects will run at
 44.1kHz (not simultaneously!), except for Reverberation, which
 just about doesn't make it. 16-Bit resolution can be achieved
 with a specially designed card.

 PC-SFX allows you to chain the effects in any order you choose,
 and also allows you to individually modify each of their
 parameters.

 This is an evaluation version of PC-SFX, and as such, is free for
 you to use and distribute as you like. However, if you do
 distribute this software, it must remain unmodified, and must be
 distributed with this documentation.

 Note that this copy has been crippled - after 5 minutes (@
 44.1kHz) or 10 minutes (@ 22kHz), processing will stop and a
 reminder message will be displayed.

The minimum system requirements for PC-SFX to run are:
-A 486 SX33 or higher processor.
-A Creative Labs Sound Blaster 16 or higher sound card.
-The MS-DOS operating system.
-A microphone, guitar or other sound source.
-Speakers (preferably amplified) or music system for sound
    output.

http://www.tc.umn.edu/nlhome/g493/swam0002/audio.html