Actually, Reason does not have the ability to record an external audio source. However, there are other ways of working with audio in Reason:

• You can record audio using another application, then export the files as WAV or AIFF. You can then load these audio files into one of Reasons's samplers like the NN-19, NN-XT, or Redrum.

*NOTE: when loading samples into a Reason sampler, use the "Browse Sample" button.

• You can run Reason in ReWire mode with a ReWire host application like Pro Tools, Digital Performer, Cubase, Logic, etc...