Practice Files

Thompson's Peaceable Kingdom can be found here (scroll down).

Midi files of Copland's In the Beginning can be found here. Either enter Copland into the composer search box or type "In the beginning" into the search query box. The top result will be SATB 1 parts. The bottom result will be SATB 2 parts.

If you want more control over the speed of playback, you can download a file of In the Beginning here (second to last item on the list). You will need to use a tool like VocisMagis to play each part separately and control the speed. More advice on playing midi files can be found here