Limit direction of "audio" in an aux connection
What you've done is a very, very bad idea. You're shorting outputs from different sources. You can mess it up big time if the two sources become active at the same time. One of them (or both) can be easily damaged.

The phone is probably detecting this condition and turning its audio output off in order to prevent that from happening. A clever little gadget, isn't it?

So I'd recommend you to consider using a switch instead. There are commercially available solutions, like this one. You'll have to look for something that's right for your intended used (you might want a switch with an additional MIC input).

If you are in a DIY mood, you may want to build it yourself. Turn to google for schematics, and have some fun with that project!