stream video with transcoded audio only with fluent-ffmpeg and nodejs

stream video with transcoded audio only with fluent-ffmpeg and nodejs

I'm putting together a simple media server to be able to serve my mkv
files to web browsers. The problem is most my files have ac3 audio and I
don't really want to recode all my files. So I'm streaming them to the
browser with ffmpeg but while doing so they all take a hit to image
quality no matter the bitrate I attempt to use.
What I would ideally like to achieve is using ffmpeg to transcode the
audio only and have the video left alone! See below for the code im
currently using:
new ffmpeg({
source: sFileName, timeout: 432000
}).toFormat('webm').withVideoBitrate(10000).addOptions(["-sn"]).writeToStream(cResponse,
function(){
console.log(arguments);
});