Bluetooth on Thai Nuvi3770

Hi,

after replacing the battery on my Thai Nuvi 3770 the microphone isn't responding when connected to my OnePlus Two mobile, meaning i can hear the caller but he doesn't hear me.

Can't find anyting on how to replace the mic, nor anything when googeling this problem.

Any help would be highly appreciated!

best regards and สวัสดีครับ :D