I have a Roland TD-6v drum modual connected via M-audio uno (midi-usb) to PC running Sonar x2. I have limited working knoladge of midi and sonar software so I'm looking for the simplest solution to the following.
The Td-6v and a few pads will be added to my acoustic kit. I want to trigger a sequence of keyboard notes by striking a drum pad. The cover band I'm playing with does not have a keyboard player. So a simple audio output from laptop to the PA should do it. I just need to assign a drum pad to start a short sequence of notes on sonar that can generate piano/keyboard notes.

You can possibly set something up under Preferences|keyboard shortcuts. In the shortcuts section, you can alternatively assign midi notes as shortcuts to do some sort of Sonar actions. I use the lowest couple of notes on my keyboard for start playback, stop playback, and pause. (I also have a whole live setup using a dedicated extra keyboard that sends all sorts of "special" midi messages, for a specialized music and computer-generated art gig thing I do.)
 
You might have to set up something where you hold down the hi-hat pedal (switched on, so to speak, sort of like holding down the control key on the computer) while you hit a specific pad to trigger the playback.
 
Alternatively, consider getting an el-cheapo mini keypad for triggering these actions.

My son has a half way decent Midi keyboard (he didn't take to guitar) but hitting individual keys while playing drums just isn't going to work to well.