If applicable switch off auto-play:
(rightclick >) Options > Preferences > General > [General settings]
(second check-box)
Load video-file from open file button or rightclick > Open file
Load external audio-file ([Shift]/[L] or rightclick > Open external audio file)
Click Play-button (or shortkey [X]).
Result: videofile and audio will start together
[If needed you can finetune external audio's sync as indicated by adicoto provided the option "Enable audio time correction" is selected under (rightclick >) Options > Preferences > Audio (5th checkbox)]. 
P.S.: I think you can also do that using an ini-file (read in doc-folder within BS.Player's installation folder, but I didn't test it now.)