C.S.Madisun

Yes. It works.

OK. So on PC, all of Half-Life's essential files are in a folder called 'valve'. That's where all the original game's models, sounds and such go.

The structure looks something like:
Half-Life\valve\sounds

The way you've got both of your mods set up, they ALSO go into that folder. And if you want them to work on PC, they absolutely CANNOT do that, as they'll overwrite the original files and you should never, EVER do that.

Instead, they need to go into a different folder with a unique name which can be loaded as a mod.

For example:
Half-Life\MetalSlugSFX\sounds

You'll also need a liblist.gam file in that directory, in order for Half-Life to properly load your mod. That's pretty straightforward to do - just copy the one from the valve directory, pop it in your MetalSlugSFX folder, open it in Notepad and change the mod name.

There's a video made by another modder that's actually really useful, as he goes through all the steps you need to make a working PC mod. Give it a watch: Youtube.com

Why do you think I have to rain on everyone's parade? :p