A little more back-up for Jingles:
Some lights can work in multiple modes: Stand alone, master/slave, sound active and DMX(or maybe just one or multiple modes).
DO NOT assume that because your light has sound activated functionality that it WILL have it in the DMX attributes. You need to read your manual and check the fixture's DMX attributes to get a clear answer to your concern.
Not all fixtures that support a sound active mode have sound active as a capability when in DMX mode. I am lucky that my lights so far do, of those that have DMX capability.
In a stand-alone sound active mode, you do NOT have DMX control.
Read your fixture's specs and manual. If you don't have a sound active "range" or "point" on your fixture's DMX attributes, then you're done, you can't do it. It's as simple as that. But if you do have a sound active range or point in the DMX attributes, again, it's as simple as that. Make sure you make it easy to get to in your profile.
Also, some fixtures that use sound active mode in DMX will also use a channel for setting sensitivity, which is often independent of anything you may have dialed-in on the internal mic's attenuator. This channel may only work this way when in sound active mode, so again, be familiar with your fixture.
And one last thing:
MyDMX has no sound input to work off of for some sort of sound active chase sequence or other things like that.