Looking for a method to provide a 5+ second waveform preview as a user plays a <video>

I'm creating a subtitle creation tool 
<https://sourceforge.net/projects/quicksubtitles/> that allows the user 
to enter subtitles and sync them to video as it plays. I would like to 
add a feature that displays the next 5 or so seconds of audio data as a 
waveform to the user to help them fine tune the syncing process as well 
as anticipate when to set an in point.

I've tried using an analyzer but it seems to be limited to 2048 samples 
which is about 120x too small. If I use an audioScript and listen for 
"audioprocess" events I can increase this to 16k but again it's way too 
small a buffer to be useful for my case.

I've also experimented with creating the entire waveform up front 
<http://jsfiddle.net/daofw1g3/8/> which works but unfortunately has 
issues with large files or longer audio clips.  I found if the video 
file is > 1GB the FIle API dispatches an "error" event when I 
readAsArrayBuffer(). So I thought perhaps I have to have the user 
isolate the audio from the but that has it's own issues as well.  I 
tested it with a 80minute 35mb mp3 and while ArrayBuffer is created but 
then decodeAudioData() never runs it's callback function. I assume both 
cases involve a memory limit?

Can anybody point me in the right direction on how I can access a large 
buffer of samples to provide the user with a 5 second or so waveform 
preview?

Thanks
Eric