You need a digital transcription program and your client will need a digital recorder. (A digital recorder can be a program or a physical digital recording device.)
One free program you could utilize to transcribe the files is
Express Scribe. Express Scribe has some great features. You can use a foot pedal, and you can also use a speech to text software program such as
Dragon Naturally Speaking.
Your client could use
Express Dictate to record the files. Express Dictate has a 14 day free trial, after that purchase price is around $60. However, any digital recorder will suffice. (The clients needs to have a digital recorder, not a tape recorder. Utilizing a tape recorder requires a physical tape to be sent by some sort of mail courier, and this takes time. Also a physical "tape" is more difficult to transcribe with a speech to text program.)
Hope this helps,
Julie