The recording will need to be done with somethign other than PHP. If you want something in the browser then I'm sure flash will have ways of recording sound input.
It could be a simple as hitting a button at the start of a transaction which begins the recording. Then hitting a stop button at the end of the transaction. This would then have produced an mp3 file, or another type of audio file. This file would then need to be stored somewhere.
The other action I would have happen is when you click the stop button an entry is made to the database which has details about the file name just captured, date - time and associate it with a transaction.
Have you looked into the legal implications of recording people?