Perhaps the simplest way is to open them in Wundows sound recorder and increase the volume before resaving. You can test them and revert the sounds to the original settings if you are not happy as long as you dont save until you are satisfied with what you are achieving.
Better still just open the offending single ambient background sound and reduce its volume. That will save a lot of work only having to edit the one sound.
If you do any of this remeber to back up your sounds first and keep copies of the original ones safe - then you can always put them back as were - if you get any problems.