MacOS – Sound output source changes based on the kind of sound

I read a post about this before on the apple discussions board. The upshot of it is there doesn't seem to be a permanent fix. However, people have provided several workarounds of which some work for some people but none seem to work for everyone. Here's a few things to try.

• Leave iPhone in Airplane mode while connected
• Turn off WiFi on the iPhone
• Turn off 3G on the iPhone
• Connect the cable while the device and stereo are off, then turn both on
• Rebooting the iPhone

I can't say from personal experience whether any of these actually work but anecdotal evidence from peoples experience in the thread linked to above would suggest they may. I think it may be a hardware design issue that is probably due to interference from the USB cable when it's connected but many people on that thread say that it only started when they updated their iPhone 4 to iOS 4.1. If you're on that version you could try updating to the latest version of the iOS software. You could also wait until Apple release iOS 5 which is likely to be October 4th 2011 as they may have fixed it.

That can not be done. There is a hardware switch in the headset plug.

It will disable the build in speakers so no software can fix that.

Apple decided that when you plug in the headset you do not want your speakers going at same time.

That is why it is not possible to use software switch.