How to select a different audio track when playing back a MKV file

[backtothefuture]

Per title,

I have a MKV file (video) whose soundtrack I don't want to use. For the soundtrack I have another file (.ac3 extension)

How can I playback the MKV file and use the .ac3 file as the audio (e.g. using VLC) ... current both files have the same file name but different extensions (i.e. mkv and ac3)

thanks