Sound Design have different meanings to different people. At Audiotuts+ they usually believe it to be “make this software synth sound like that“. Yes, the focus is mainly music. But every now and then there’s an interesting article about sound effects. If you want to access all the material you’ll need a payed subscription, but they provide many tutorials for free.
Stephan Schütze not only provides the world with 20,000 sounds for free, he also writes tutorials for audio production with a focus on sound design. The difficulty level ranges from basic to advanced. On the site you’ll find everything from Audio Editing 101 to Creating Explosions to basic implementation in FMOD.