The word transliterates from the arabic جنبية as janbiya but is pronounced as jambiya. The root JNB means by or side or beside. Watch the attached video and you will hear the pronunciation with an M. I have heard it is just something idiosyncratic in the Yemeni dialect. When they pronounce the plural it is with an N, janaby. I have heard it pronounced with an M since I was a child.
http://www.youtube.com/watch?v=N_DK2DmlsXE
Just to make sure I was not remembering this wrong I double checked with former ambassador David Newton who was stationed in Yemen on and off since 1966.
"Hello Michael. You are correct. The word is written with an "n," but it is pronounced with an "m," simply because of ease of speaking. The two letters "b" and "m" are spoken in the front of the mouth. whereas "n" is pronounced in the back of the mouth. So "m" is much easier to use next to a 'b." I'll look at your pictures. Regards. David"