How to combine audio without a mixer?

I’m looking to combine a bunch of inputs to 1 output, so far I’ve managed to do it by using a 5 way splitter cable and just …putting them all in. But now that i have around 4 out of 5 cables used the audio is getting quieter with the lack of power. Is there something that can combine this audio? I can use a mixer since most don’t have rca and I’m not looking for adapters plus I feel like mixers would be too big for me to put them anywhere. I was possibly thinking of using a line mixer but the one I got the cheap Behringer mx400 produces mono audio…which sucks. Is there some solution to this?
VidMate Mobdro