First one:
On the first one you're doing it right. Some systems (like yours) don't seem to like the sound system, disable sounds in config/IC2.cfg and try again.
It works . I want to know whether this is a IC2 or lwjgl bug (newest lwjgl version - I use lwjgl for my 2D RPG game, too).
I look inside the IC2 jar archive and found some ogg/vorbis files. Does IC2 use the experimental AL_EXT_vorbis extension of OpenAL via lwjgl?
AFAIR I had no sound from IC2 at MC 1.1 in the past. I recommend the jorbis library (http://www.jcraft.com/jorbis/) to you.
But minecraft crashes reproducible at closing with the same Windows crash dialog, seems like option "soundsEnabled" is not so good implemented.
Second one:
QuoteThat's imo not even related to sounds. The crash log refers to a
signature mismatch due to a modified minecraft.jar. You'll have to
delete the two MOJANG_C signature file in the META-INF subfolder inside
minecraft.jar.
[It's] trying to use 1.2.4 Forge in 1.2.3
I deleted the META-INF subfolder inside minecraft.jar before adding Forge. I tried every Forge version from v1.4.1.59 down to v1.4.0.52 (all versions for MC 1.2.3).
v1.4.0.52 was the first one that works instead of a blackscreen. Look at http://www.mod-buildcraft.com/forums/topic/forge-downloads/ ; Forge v1.4.1.X is for MC 1.2.3 (ML and MLMP installed with required version), but all Forge v1.4.1.X releases force MC to blackscreen - seems like net.minecraft.client.Minecraft is broken in those Forge releases.
PS: I hope that you port IC2 to MC 1.2.4 (Forge 2.0.0.X) soon.
PPS: Texture of reinforced doors looks strange, above block: plain violet, below block: green looks like wood.