questionsdear audiophiles: how to make the rear speakers…

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If it's a home-theater-in-a-box type, then don't bother with my suggestion. Otherwise, the only "about $50" solution I can think of is this, and it will have limitations and subject to adjacent radio stations.

Convert rear channel outputs to line level with this:
http://www.amazon.com/Audio-Systems-Level-Converter-B65N/dp/B000LP4RMQ

FM transmitter + you'll have to repurpose an old 5v wall-wart to ditch the batteries:
http://www.amazon.com/LineX-LinkPlus-Wireless-Transmitter-Adaptor/dp/B000SZAKKM

Receive it at the back with this amp on whatever frequency your transmitter is set at:
http://www.amazon.com/Lepai-LP-A68-Digital-Amplifier-Remote/dp/B00721Z1AE

(Cables and speaker wire to connect converter to transmitter and to reconnect your existing speakers not included in this; figure those out yourself.)

vote-for2vote-against

Do this only if your carpet is like this:

One more thing for hiding wires is to go under the carpet. This is what I did, but I did install carpet for 2 years and know the mechanics of the carpet, tackstrip and pad. Be careful for pad roll if you try this.
Get some fish tape.
pull up 1-2 inches of carpet along the wall of where you want to run the wire. You want a direct path to your back speakers with little to no traffic. You will remove all funiture in this path temporally. Bend the fish tape so it points up at the end like this, perhaps at a 30 degree angle_/ to keep from snagging on the pad. The tap will push up on the carpet as you run tape. Once on the other side tape the cable to the fishtape and wrap it in plastic wrap to smoothly pull back (you should pull both wires at the same time)
I did run mine in moderate traffic with no issues, the wire is very low voltage and sinks into the pad.

vote-for4vote-against

Simply put, you don't. Right now there isn't really any good way to do it. Your options are:

1) Hide the wires. If you are very luck, and either have the funds to hire somebody to do it, or are handy enough to do it yourself AND you have the correct setup in your HT room (either with an attic above or crawlspace/basement below) you can run the speaker wires from the AVR, to a wall plate, then to a wallplate (or floorplate), then to a speaker.

2) If that isn't an option, and running the wires in conduit is not a good option (for whatever reason) you can consider a soundbar. Usually they don't do a very good job with rear surround, but it is better than nothing.

vote-for8vote-against

Wireless systems often have lag associated with them. Usually, I'll tuck the wires into the baseboard and run the wire around the room, taking the long route, and you don't even notice.

If you really want a decent wireless system, it will cost you beaucoup bucks.