bon-swa
Here's a breakdown:
* bon is pronounced like "bon" in English, with a short "o" sound.
* soir is pronounced like "swa" in English, with a short "a" sound.
The "s" at the end of "soir" is silent.
So, the entire word is pronounced with a rising intonation, starting with a soft "bon" and ending with a slightly stronger "swa".