Don't know what soundpack you downloaded, but here are the "basic" instructions take from Mitsu's sticky post ...
This file is compressed as ZIP, after downloading, decompress it and cut and paste "sounds" folder to Aces High II directory.
Do not overwrite the folder, please delete or back up your original \Aces High II\sounds\ folder before pasting.
I recommend you change original sound folder to "sounds_b" or other name.