Relevant to Blender v2.31

Since Blender 2.28 there is a (still limited) Audio sequencing toolbox. You can add WAV files via the SHIFT-A menu and selecting the Sound entry.

A green audio strip will be created. No 'high level' mixing features are present currently. You can have as many Audio strips as you wish and the result will be the mixing of all them.

You can give each strip its own name and Gain (in dB) via the NKEY menu. This also let you set a strip to mute or 'Pan' it; -1 is hard left, +1 is hard right.

A 'Volume' IPO can be added to the strip in the IPO Window as it is done for effect strips. The Fac channel is the volume here. IPO frames 1-100 correspond to the whole sample length, 1.0 is full volume, 0.0 is completely silent.

Blender cannot yet mix the sound in the final product of the Sequence Editor. The output is therefore a video file, if the ANIM button in the Anim Panel of the Scene Context/Render Sub-context is used as described before, or a separate WAV file, containing the full audio sequence, in the same directory of the video file and with the same name but with a .WAV extension. This audio file is created via the MIXDOWN button in the Sequencer button of the Scene Context, Sound Sub-context.

You can mix Video and Audio later on with an external program. The advantage of using Blender's sequence editor lies in the easier synchronization attainable by sequencing frames and sound in the same application.