Apps have limitatins
:ipad-keyboard:Video recording apps like iMovie, Splice record audio as well as video so the new drum sound would be captured with the video but however you cannot have that same app recording the audio output of another app say your music player on your iPad. The reason for this is that when an app loads into your iPad that uses the Audio input like the mic it mutes the output so that you don't get feedback from the internal speaker. This is a protection written into the APIs from the iOS SDK code. This loop cycle would destroy any recording so that protection cannot be by-passed. The only way th accomplish this is to play the pre-recorded music out a small stereo placed near the iPad so that the movie recording pics boyhood your new drum sound and the recording your trying to play to. Try placing the the speakers in different places in the room to find the best mix. Record multiple videos of these changes and choose the best sounding. That's all you can do with this idea you have.
