Articles
The traditional music of Thailand
A basic overview, written by Ingo Stoevesandt
 
http://www.paradisemoon.com/thai_main/Huahin/thai_instruments_intro.htm
A nice overview of Thai music written by Greg Turner 

http://pioneer.chula.ac.th/~sbussako/
The pages of ethnomusicologist, artist, composer and researcher Professor Bussakorn Sumrongthong

http://www.asiasound.com/content/Learn/Timelines/thailand
A good small overview of traditional Thai music with audio files and book list

http://www.seasite.niu.edu/Thai/music/default.htm
A huge portal for every kind of Thai music

http://www.thailand.com/travel/arts/art_music_classic.htm
A short introduction into traditional Thai music

http://www.sriwittayapaknam.ac.th/multimedia/index.html
Some flash files (audio and video) about traditoinal music from the Sriwittayapaknam school

The traditional music of Northeast Thailand
An article by Terry E. Miller

http://www.asianclassicalmp3.org/lanna.htm
Some big audio files from the ensemble tradition of the Lanna Kingdom + even more links