It's still possible to modify your SC2 sound (before it was as easy as dropping a folder into your SC2 directory), although it takes a little more work. I'm going to post a little guide here for those who aren't familiar with working with MPQ's. You can replace any sound in the game using this method. Here's an example:
Do this all at your own risk. Editing MPQ's requires some competence with computers and is most definitely against the ToU/EULA. I'm not responsible for whatever happens to you, your account, or your installation.
1) Open your SC2 directory.
2) Navigate to Mods\Liberty.SC2Mod.
3) CTRL+Click enUS.SC2Assets and base.SC2Assets.
4) Copy and paste them to a safe folder (this'll take a few minutes).
(note: this step is to ensure that should anything go wrong with your editing, you will have the original MPQ files to work with. I left the sound edits intact when I installed the most recent patch and my installation is still alright, but this does not mean it will be the case for future patches (though I suspect it will be).
5) Download an MPQ editor. I recommend the one found here: http://www.zezula.net/en/mpq/download.html
6) Prepare any sound files you want to use in the swap. If your file has the wrong extension, Audacity (http://audacity.sourceforge.net/download/) is a great free tool you can use to easily record and edit audio files (you'll also probably need it to raise the volume on your audio since otherwise it's too quiet ingame).
I'll be writing the rest of the guide to walk you through applying the SC1 sound patch found here: http://www.teamliquid.net/forum/viewmessage.php?topic_id=117317
1) Fire up your MPQ editor and navigate to Liberty.SC2Mod\base.SC2Assets. Navigate in the side bar to Assets\Sounds\Protoss\Archon.
2) Open the sound pack. Navigate to Assets\Sounds\Protoss\Archon.
3) Drag all the files from the folder to the MPQ editor. If you did it right, a popup saying "Rules for adding files to MPQ" should appear. Just click OK. Afterwards, click "Yes All" (the old files are being overwritten).
4) Repeat for all the other files. Everything in the Assets\... directory is in base.SC2Assets. Once you're done with that, close it in the editor (it automatically saves changes). Everything in the LocalizedData folder is handled in enUS.SC2Assets (or the appropriate langCOUNTRY combination for your localized copy).
5) Be sure not to launch SC2 until you have completely closed your MPQ editor. If you've done it correctly, the sounds should be swapped.
6) If you want to use your own files not included in the sound pack, be aware that in order for the swap to work, the replacement sound file must have the same name and extension as the original one. This means .mp3's won't work where .ogg's are needed, etc. If you need help on this, go ahead and post in the this thread or PM me or whatever.
Hope you all enjoy! I've been running SC1 sounds for a few days now and it's so nice to hear the old marine guns again.