How can I stream all audio from my Mac to my Android phone? I'm sick of switching my headphone jack from my Mac to phone and vice versa. Is there any way to stream all audio playing on my Mac to my Android phone, so I can hear any audio from either my Mac or my phone through the same pair of headphones, at the same time?
Rogue Amoeba's Airfoil will do what you want. Run Airfoil on your Mac and the Airfoil Speakers app on your Android device.

Alternatively, there may be free solutions. The protocol used by Apple devices to send audio to e.g. AppleTV and Airport Express devices is called Airplay and has been reverse-engineered by projects such as Shairport. There may be an Android app which emulates this protocol to make your phone into an Airplay destination. However you'll have to search carefully as the more common implementation is to enable the Android device to send audio to the AppleTV etc.