Slooooooooow down. Take a deeeeeeeeep breath.
I don't think I understood a word you said, but from what I THINK you said, I'll offer this advice.
First off, turn off all power to all device involved here. Never kerfutz around with this stuff while it's powered up. It might stop working. ...trust me on this.
Then, forget what you have going now. We're goin' back to page one. If you have any questions on how to do what I'm suggesting, refer to your owner's manual.
Unhook the RF mod from everything except for the 75 onhm cable going to the TV.
Now, we'll hook up your X box and DVD.
Run a digital audio connection (coax or toslink, it doesn't matter)from your X box to one of the available inputs on your receiver. Now run a video connection from the X box to the corrosponding input on your receiver.
Now, do exacty the same thing for the DVD that you did for the X box, but going to a different input on the receiver.
Now for that RF mod... connect the yellow composite video that would normally go from the receiver to the TV to the yellow jack on the RF mod. Do whatever you have to do to get a picture on the TV from these guys.
Yeah, I know the red/white to the RF Mod isn't hooked up yet. We'll worry about that later.
Make sure that the receiver "sees" both these devices. When that's done, use the receiver to "select" these devices and play something thru them. You should now be seeing the video from these on the TV and getting the "digital" sound from these devices thru the speakers that are connected to the receiver.
You've now got full functionality of your system for video and sound thru the receiver.
Now, we'll see about the red/white audio cables to the RF mod. Here's where it gets hazy....
Look carefully at the back of your receiver where the yellow cable that goes to the RF mod is. It's probably labeled "monitor" or some such nonesense.
Continue looking (or check the manual) for a pair of red/white audio jacks that corrospond to this. If you find them, hook them up to the red/white jacks on the RF modulator.
This should now feed whatever you select from the receiver to the Rf mod, and thru to the TV, both video and sound.
Hopefully, your receiver will allow you to mute the speakers (or you could turn it all the way down) when you want to hear thru the TVs speakers. The otherside of this coin is that you'll have to either mute the TV speakers when you don't want to hear 'em. ...either that or turn 'em all the way down.
If you can't find them or you don't have' em, then I dunno what else to tell ya short of saying that you'll have to hook up the red/white to the RF mod manually for whatever source you choose.
Again, learn to love your manuals. You're gonna need 'em. YMMV