CDs encoded in ogg format, at lowest bitrate possible.

birds:
Birdsongs_PNW - Birdsongs of the Pacific Northwest, by Stephen Whitney; 1 CD
Know_Your_Bird_Sounds_Common_Western_Species - Know Your Bird Sounds: Common Western Species, by Kevin Colver and Lang Elliott; 1 CD
Songs_Wild_Birds - The Songs of Wild Birds, by Lang Elliott; 1 CD
Stokes - Stokes Field Guide to Bird Songs: Western Region; 4 CDs
Western_Bird_Songs - A Field Guide to Western Bird Songs (Peterson Field Guides); 2 CDs

Frogs & Toads:
FrogsToadsNA - The Frogs and Toads of North America: A Comprehensive Guide to Their Identification,Behavior, and Calls, by Lang Elliott; 1 CD (the book accompanying this CD is included in Nature Field Guides #1)

insects:
The_Songs_of_Insects - The Songs of Insects, by Lang Elliott & Wil Hershberger; 1 CD (the book accompanying this CD is included in Nature Field Guides #1)