I'm looking for the simplest way to record voice -- a two people conversation -- with a Macbook Air. The goal is then later to listen to it on the Mac and transcribe it.
Scenario 1: You're both in the same room
Open up QuickTime Player. Select
File -> New Audio Recording.... Make sure the source is set to Built-in Microphone (see screen shot below). Hit the big red Record button. Talk away.
Scenario 2: You're using Skype or something like it to converse
This is a little trickier but not impossible. You'll need the SoundFlower app from Cycling '74. If you're using Skype you'll set it's audio output to be the SoundFlower input. Then in QuickTime you'll set the audio input to be SoundFlower's 2-channel output. This routes the audio from Skype in to QuickTime. Record as above.
The simplest way is to use either Quicktime (simply choose File>New Audio Recording) or a free and open-source app like Audacity. Your MBA has a built in microphone, so all you need to do is click record and then start your conversation.
If you do this regularly, it way be worth investing in a USB mic that has better sound quality than the built-in mic on your Macbook. Something like the Samson Go Mic is fairly well regarded for field recordings, dictation, and interview recordings.
Lastly, transcription can be a huge pain without software that supports easy pause/resume/rewind capabilities. There are a couple free ones out there, like Transcriptions or more expensive commercial solutions. (Hint: Get a foot pedal!)
Sound-to-text technology is getting increasingly more accurate (though not foolproof as anyone who uses Google voicemail transcription knows), so depending on the quality of your source material and the time you spend training, you can also eliminate a large chunk of the manual transcription with something like MacSpeech Scribe
Open QuickTime Player and select File -> New Audio Recording from the menu. This is a new feature (not sure whether it was in Lion, Snow Leopard, or an independent QuickTime update). Previously your options were: