Minimising data and bandwidth needed in web apps

I've been persuaded by a wise colleague that users generally don't like downloading apps on their phones, so I'm planning to make my new Hype-based audio project as a web app rather than go through all the hassle of importing into Xcode. Optimise for users and not for the App Store is the idea.

And to minimise data usage by participants - and my bandwidth - it needs to be lightweight.
The problem: It will have 60+ minutes of mp3 audio, albeit in small chunks.

I will use howler.js with Hype because I find it quicker and easier to make the audio skip forward and back.

The key thing is how low can you go it terms of mp3 quality and still retain good listenability with spoken word. Anybody got any thoughts on this ?

I think you could get away with 128 kbps / vbr MP3. This will sometimes dip below 128 and should be suitable for voice. Streaming audio at this rate should be possible on even pretty slow connections. Unless you have a specific target demographic who has low bandwidth mobile plans, that quality / bitrate should be just fine.