I feel like your question is pretty skewed because it assumes that techno IS the sound of post-wall Berlin.

What do you mean by "the sound of", Mainstream? a part of the cultural identity of Berlin?

Techno in itself is NOT a post-wall creation, and some would argue that techno was not born in Berlin.

Then, did Berlin have a distinctive "sound" in post-wall era, and was it techno?

When would you say the "post-wall era" began and ended?

Do you just mean: How did techno become so popular in Berlin?

I do apologize for the nitpicking as i would like to try and answer your question.

Edit: Meanwhile, let me recommend some solid documentaries on techno and on music in Berlin.

Krautrock - The Rebirth of Germany (BBC documentary)

High Tech Soul - The Creation of Techno Music (History of techno)

Children Of Berlin (2010 documentary on the Berlin techno scene)

Don't Forget to Go Home (2006 documentary on the Berlin techno scene)

Universal Techno (1996 documentary on the Berlin techno scene)(German with French subtitles)

MFS Berliner Trance (1993 documentary on trance music in berlin)

The Story of Tresor (Documentary on Berlin's Tresor club)

The Berlin University of the Arts hast just held a symposium on the "Aesthetics and historiography of electronid dance music". The German version of that page lists several German news reports the most interesting of which is the one from Die Zeit. There's also a russian one.