Siri only works when use raise to speak-feature

Siri works as in the icon shows up when I press the home button and it starts "listening". But the microphone does not show any movement. So Siri "doesn't hear me". Funny though: When I just pick up the phone and talk, she understands me perfectly.

- Siri via home button does NOT pick up my voice

- Siri with "raise to speak" enabled works

- Microphone works

- no network problems (my iPad-Siri works fine)

Kinda annoying. Things I've tried:

- reboot

- turn off/on

- diable bluetooth

Hardware:

- iPhone 5, 64 GB, white (as if it matters *g*).

I am kind of clueless. Any help? Anyone? Thanks a lot.