Using diodes to mix 2 audio signals into one set of speakers

[nengguo]

It's my first post in here and feel a bit of nervous there will be some mistake in my post, even though I have passed the forum rules, I'm tried to avoid it, I hope it will be ok.
The situation is following: I have the audio output of 2 computers connected to only one set of speakers.
Each computer uses one of these external USB sound cards:

And I use this splitter cable to wire the signals together:

The whole thing works, but with some minor problems:

There's a little bit of noise when the speakers are set at max volume
One audio signal sounds a bit louder than the other
When recording the microphone input, sometimes the audio output of the other computer becomes the mic input
So obviously the two signals are interfering each other and also each signal finds its way into the other sound card because the splitter cable is meant to be a splitter and not a mixer.

So I thought if I could solve this by adding diodes to each side of the splitter cable so that each audio signal goes to the speakers only and not to the other sound card.

Will this idea work?
What kind of diode should I use for that?
Thanks all guys.

 

[jemplayer]

I would just get a cheap used mixer from guitar center. That way you can balance the volume.

 

[Dave L]

You shouldn't sum signals passively like that to begin with, it´s one of the few instances where you actually can harm equipment if you´re unlucky. I´d second the notion to get a small mixer.

 

[CrazyNutz]

Dont use diodes.

You need to use resistors like this:

Try 10k or 20k

 

[scottosan]

It may not be worth the hassle. Keep in mind the speakers operate on AC