Your best option is .mp3 - I mean I have both 'War of the Worlds' albums (Jeff Wayne) as .mp3 on a CD, and they work out fine.
You can buy .mp3 compatible DVD players for about £30 - cheap DVD players make much more sense than expensive CD players.
You can get portable CD players that support .mp3 for £25.
I reckon a MP3 CD could hold a good 8 hours of audio no problem. You would be advised to keep them split up into chapters or something though, then let the CD play them in sequence as well as letting the user select a chapter - don't let it make a massive single .mp3 file or a lot of players will choke on it.
Van-B