This falls under the category of 'things I've known about forever and consistently forgotten to blog about': FSI-Language-Courses.com.
If you're not familiar with it, FSI is the Foreign Service Institute, the U.S. government's training center for foreign service officers—diplomats and the like. It was established 60 years ago to replace an earlier incarnation established in 1924. The Institute has developed language courses in a large number of languages. And as a government body, all of its work is in the public domain and freely distributable.
FSI Language Courses was started by Glen D. Fellows in 2006. Users on the site (including Glen) scan FSI coursebooks and record FSI audio tapes that they either buy (they're regularly sold by third parties at exorbitant prices) or check out from a library. The resulting pdf and mp3 files are then made available for everyone. No copyright. No cost. Simple. Brilliant.
The site has FSI course materials for 34 languages at the moment, including Thai, Lao and Cambodian. Granted, these courses are at the youngest a few decades old, but you can't beat the price.
The site has the following materials for Thai:
Introduction to Thai phonology (19 mp3 files—3.5 hours)
Thai Basic Course Volume 1 (student text pdf—426 pages; 20 mp3 files—10 hours)
Thai Basic Course Volume 2 (student text pdf—421 pages; audio needed)
And these materials for Lao:
Reading Lao (student text pdf—492 pages; 79 mp3 files—35 hours)*
Lao Basic Course Volume 1 (student text pdf—448 pages; audio needed)
And materials for Cambodian:
Contemporary Cambodian: Grammatical Sketch (student text pdf—125 pages)
Cambodian Basic Course Volume 1 (student text pdf—453 pages; 45 mp3 files—12.5 hours)
Cambodian Basic Course Volume 2 (student text pdf—367 pages; audio needed)
Among the other languages you'll find materials for are Mandarin (listed as "Chinese"), Cantonese, Korean, Hindi, Swahili, Arabic, and a couple dozen more. Should keep any polyglot busy for years, really.
And if your eyes are rolling back in your head at the thought of right-click downloading all those files, then try DownThemAll, one of my favorite Firefox plug-ins. Just play nice and don't download everything at once.
*There is a typo in the URL for Tape 39 of Reading Lao, but you can manually correct it: change 396A to 096A in the filename.