The xpSound modifier is slightly different to most other modifiers in that it can do two things.

It can change the emitter to influence emission rate and change particle parameters when they are first created.

It can also change particles during the course of the animation as with any other modifier.

General tab

xpSound General tab menu.

Enabled

Checking this box enables the xpSound modifier.

Mode

Set at Independent by default.

You can change this to Action-Controlled.

Independent Mode

In this mode, particles will be affected if they come into the field of effect of the modifier.

Action-Controlled Mode

When in the Action-Controlled Mode setting, the modifier will only act on a particle when told to do so by an action.

Sound

The sound file to use.