The DTMF decoder doesn't care where the signal comes from as long as it has a sufficient amplitude and good signal to noise ratio. A microphone can be used, you will need a pre-amplifier to boost the microphone's weak signal to a level that is suitable for the decoder.
Note that when you pick up a DTMF signal with a microphone you will pick up noise, to (e.g. environmental sounds). Depending on the level of the noise it may be hard to reliably decode teh DTMF signal.