Android Audio Recording -- 2

I have an android mobile application that allows users to listen to instrumentals from other users. The objective for this project is to create an android activity that allows a user to record to a selected track. Once the selected track is recorded I’d like the freelancer to merge the two tracks together into a single audio file. Currently when the user selects the song they want to record, the selected audio file will be downloaded to the android filesystem. The downloaded file will be an mp3 file.(I will provide a file to test with) I want the activity to have a recording button, stop button and a play button. The recording button will arm the downloaded file and start recording from the devices’ microphone. The stop button will stop the playback of the instrumental and the recording. The play button will playback the recording. Expectations: I expect 0 latency when the audio is recorded to. I expect the mixed file to have the duration of the longest of the two files(instrumental and recording).