NW-A45 guide
Music
The NW-A45 can play MP3, AAC, and FLAC files. I can personally attest to it being able to keep track of over 10,000 tracks stored on an external microSD card, with no problem. It takes a short while to boot up and catalogue them, but it's designed to stay on for days at a time, so this isn't much of an issue.
Playlists
To add a playlist, create an .m3u file. (I made the directory /MUSIC/Playlists to put mine in.) The name of the file, excluding the extension, will automatically be the name of the playlist.
Edit this file as plain text, with one track per line. You should reference the full path, starting with /MUSIC. You don't need to backslash-escape spaces.
You can't add whole directories at once, only a single file at a time, one per line.
For example, my /MUSIC/Playlists/Selected Ambient Works +.m3u file contains the following:
/MUSIC/A/Aphex Twin/_..I Care Because You Do/18 consciousness utopia.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/01 Xtal.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/02 Tha.flac /MUSIC/A/Aphex Twin/[user48736353001]/22 Aphex Twin - Sams Car.mp3 /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/03 Pulsewidth.flac /MUSIC/A/Aphex Twin/Classics/09 AFX - Analogue Bubblebath 1.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/04 Ageispolis.flac /MUSIC/C/Caustic Window/Caustic Window LP/09 squidge in the fridge.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/10 Ptolemy.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/07 Heliosphan.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/08 We Are the Music Makers.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/12 Delphium.flac /MUSIC/A/Aphex Twin/Selected Ambient Works 85-92/13 Actium.flac
Misc guides: IRC guide | NW-A45 guide | Regular expressions guide