During Martch Microsoft released another build of IE7 (build 5335). After installing this build Pandora stopped to play music. Read my post.
Today I have installed beta 3 of IE7 (build 5450) and the music returned. I don't know whether Pandora developers fixed the problem earlier or I should "blame" Microsoft for this.
But now I can proudly inform that Open Pandora is compatible with Microsoft IE7 beta 3.