By investigating the sound problem in CAOD, I have found another trick for the game where you can enable the Sub XXI for single missions:
Go in the menu for single missions, chose convoy encounter, make your setup that you would do with a type XXI u-boat (but you have to chose another one, the XXI is not in the menu), then start the sim and quit.
Go open your file SIMIN.DAT with a hex editor, and change the value of Hex adress 41 to the value 07 (which is for the XXI).
Restart game and single mission menu for convoy encounters, you will see the tab for the sub choice empty - leave it so, don't change it - make other settings, chose your realism and go! You will play with the Type XXI
Now, I want to correct a statement in the Tips section of Subsim about CAOD:
A Sierra developer has said that the Sonar of the type XXI is broken. That's true for CAOD, but not true for the upgraded DOS version, here it works as intended.
You use the XXI sonar by chosing a red line which is a passive sonar contact, the line will turn blue, and you hit the sonar icon on the map. It will give the range of the target. If you go to TDC you will see this range correctly entered with the bearing of the target, that's all you need to fire an acoustic torpedo to hit the target. For the sonar to work you have to be in front of the target at +/-140° at lower speed, your baffles are blind.
You can fire acoustic torpedos Typ V or LUT even if you are at up to 100m depth. Other torpedos should not be used with type XXI, because they will only 'run straight' without any acoustic guidance, so they are useless under water unless you are a genius of a sub commander...
This also means that, alas, type XXI u-boats are useless in CAOD, if you cannot fire torpedos underwater then the whole sub is useless, except its very high underwater top speed of 17 knots and 14 knots full speed. You will have to play the DOS version
History:
What I say here has never happened in WWII, the type XXI just did a 'simulated' attack against a warship before surrendering West of Norway - it was the end of the war...