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The CAT 5 100 meter (~300 feet) limit refers to operation of 100 MBit ethernet connections. The limit is imposed simply to say the connection exceeding that length does not meet TIA 568 spec for error free data communication.
All of the CAT type cables have the same distance limit they just run at different data rates.
You will be running base band audio so the 100 meter limit would be meaningless. However, there would be copper loss as RFB pointed out due to the DC resistance of the wire.
The balun may step up the signal voltage into the cable like a transformer. This would reduce the copper loss depending on how much the signal voltage is raised (higher voltage, lower current.) On the receiving end the balun would step the signal voltage back down. This may be how they advertise such extraordinary transmission distances.
