No, it's been discussed before. It's probably hardcoded and can't be changed, but there are reasons for that. The thinking for SH4 was that US subs had a retractable sonar installation under the keel. U-boats did not. A u-boat's hydrophone array was in the bow of the boat, and when it was surfaced the array was right where the most turbulance was created. The rotating sensor was up on the deck, so when the boat was surfaced it was up out of the water, and useless.
My objection to having even US boats be able to hear on the surface is that their sonar array was very much like that of destroyers. If a sub can hear a destroyer or a merchant while surfaced, why can't a destroyer hear a surfaced sub? I could easily be wrong, but Outside of SH4 I've never heard of a sub being able to use sonar to hear surface ships while surfaced themselves.
Update: It seems that the underwater array was active SONAR only. The passive array, or hydrophone, was up on the deck exactly as in the u-boats. They could not listen while surfaced.
http://maritime.org/doc/fleetsub/sonar/index.htm