Oscilloscope preamp for smartphones
The microphone input will be sensitive to signals of millivolts, and definitely should be limited to 1V or less. It will probably be AC coupled internally.

See http://www.instructables.com/id/A-Preamplifier-for-Smartphone-Oscilloscopes/
and http://hackaday.com/2012/07/14/android-oscilloscope-built-from-parts-just-laying-around/ ; the latter of those is just an attenuator, relying on the built-in preamp. The former adds a unity gain buffer as an impedance converter, and will probably produce better results. In both cases it's only suitable for audio frequency ranges.