This is done using DOSBox v0.74-3 along with DOSBox Game Launcher v0.96 and the settings are all done in DBGL not DOSBox itself.
I ran into an issue setting this up and wanted to share with you because the graphics will start out in a larger window and then will shift to a smaller window once the actual game starts.
These settings work great and result in a large window for the entire game experience with no video or audio issues.
Output: opengl (nothing else I tried forced the actual game window to be as large as the opening screen)
Frameskip: 0 (default)
Scaler: normal2x (default)
Forced: checked
FS Resolution: 1280 x 960 (exact doubling of 640 x 480)
Window Resolution: 1280 x 960 (exact doubling of 640 x 480)
Fullscreen: unchecked (default)
Double Buffering: unchecked (default)
Aspect Correction: unchecked (default)
I did use LemmingsCorrected.zip from here:
https://www.camanis.net/lemmings/lemmingswelt/I don't think using that was necessary and the settings above should work with the original un-corrected version.
Steve