How does WaveMux work?
SoftCam operates as a virtual video capture device. WaveMux is a virtual sound device. They use Luminositi's patent-pending MediaCortex^™ technology to mediate audio and video between applications and the physical or stored sources (cameras, sound cards, or stored media files).

When I try to play a sound effect while I have a Title playing, I can't hear anything.
WaveMux blends only one sound source with your voice at a time. If your Title has any sound associated with it already (such as the audio track of an AVI file), then ad-hoc sound effects cannot be played.