Within a technology-enhanced learning environment, what tech devices and their attributes (such as the use of YouTube or other emerging techs) would make for an appropriate selection for sound design in technology-enhanced learning for middle school students?

There are many ways to use technology to enhance middle school learning across the curriculum and to differentiate teaching strategies so that students are given different methods of learning and various ways to show what they know. Differentiated learning is the basis of sound teaching.
For example, the Edutopia article in the link below discusses the use of a tool called Photo Story, which is part of Windows. This tool allows students to create pictures, and they can also add sound effects. The article recommends this tool for the writing of dialogue in stories, and it might also be used to help students hear and comprehend sections of books or plays. By hearing the words, students who learn better by hearing can have a way to access the reading that is tailored to their learning style. Students who learn by seeing will be helped by creating pictures. Students can also use Garage Band to write and record a song that encapsulates part of their reading or that summarizes an episode in history. 
Simple blog posts, which students can do on Word Press, can also help students learn from other students. For example, students can post their answers to questions or can comment on readings in different areas. Then they can read their fellow students' posts. In this way, they can create a dialogue that fosters learning outside the classroom.