As an owner of the MyDMX interface and the Elation SD+ interface, let me say that I've also done EXTENSIVE documentation on the MyDMX and MIDI for "sync" type purposes, and have done a MAJOR show using Compu Show that required extensive sync as well.
Compu Show can be an all-in-one solution. You import the audio file, then start landing your scenes from Compu Show where you need them to be. You can also used something called switches, which are basically "sub-scenes" that you can use to modify existing scenes, or can be a scene in and of itself. Compu Show is a significantly more complex product that MyDMX, and as such, comes with a higher price tag. There is a steep learning curve. Do I recommend Compu Show? ABSOLUTELY!! Also, Compu Show can work with MTC.
MyDMX is much simpler. To effectively do what you think you want to do, you need to use two computers, mostly for the DAW-type functionality of doing a backing track(or tracks) coupled with MIDI. Sonar Home is your cheapest route at around $100, and yes you need a MIDI interface on each computer involved, which can add at least another $60, but most likely you'll go with a 2-bank, and that could skyrocket your costs to around $160 or so for a pair. You don't need top of the line, you just need rugged and reliable.
It ultimately comes down to "what is the right tool for the job". While you CAN do this in MyDMX, really, it's not the right tool for this task. You want Compu Show.
MyDMX is designed more for DJ's who want and need a simple interface for rapid scene calling. I'm not degrading DJ's here. Between queuing and the other stuff they are doing, the last thing they need to throw into their mix is an interface that one could get lost in quickly. While Compu Show is easy to navigate, in the fast and furious world of spinning music, you don't have time to deal with it, and MyDMX works better.
You might be thinking, after hearing prices, that how can one justify such a price for Compu Show? Well, I can, and do it simply.
1: I work on 2 productions that use extensive sync. Couple that with rehearsal stuff and the bouncing around, Compu Show naturally caters to that.
2: If I were to start designing lighting shows for my karaoke library for weddings and what-not(not gonna happen, but bear with me), I could design lighting designs for each song and have a fairly quick recall.
If you're making your playlist sets, you could quickly roll up, set your rig, run it and go hit the bar while the computer does the work. Of course, if you're reading the room, this might break down a bit. But if you're working off a rigid set-list, this can be ideal for you.
Then again, for live shows, I did an event using Compu Show, and if I compare between the two(Compu Show vs. MyDMX), Compu Show needs me to do a bit more work because I did better scenes in MyDMX right now. With practice, Compu Show can rock an event live, provided the lighting designer did a good job.
But for fast and easy access, MyDMX kicks butt. Affordable with TONS of features for the price, but it does have limitations. You can't give it all away at that price.