Jams is a free, powerful and elegant music player for Android.
RhombusLib is an Android library project that decodes magnetic stripe audio data. It is designed for use with the unencrypted SquareUp credit card reading dongle, though it should work with any mechanism to get the raw data from the card as audio.
RhombusLib can decode either track 1 or track 2 data, and can even determine on the fly which one to decode to. It supports swiping in either direction.