Cable to send audio from iPhone to android phone via TRRS
I understand this question has long since been asked. However I have useful information for Google-fu students and teachers.

From what I understand, iPhones by default have the TRRS 4-conductor jack. Androids, on the other hand, vary in specifications.

Quoted from here:


  I'm sure android and apple are the same kind (I've used them interchangeably) though samsung android phones have two rings swapped.


More specifications related to Android, specifically the Nexus One, can be found here (with wiring diagrams). However, the application is different than what you are proposing. Still, this may help.