Mircophone Problems

[rockmanX69]

I am taking two sets of RCA cables into a single input in a receiver. One is an 1/8" to stereo RCA from the computer that music audio is coming from. The other is a stereo 1/4" to stereo RCA from a 4ch Behringer mixer, which is the mics. When the fed is just the RCA from the mixer the audio levels are normal. However when I use a dual RCA to mono RCA so that both signals can be in one input for the receiver the audio level of the mixer drops off dramatically. Any suggestions or other pieces of equipment that can be put in line to boost the level of the mixer. Thank you.