FCP 7 and 32bit audio needs constant rendering

Audio files in my sequence need constant rendering.

I am working with two types of footage -- MP4 and MXF. I converted the MP4 files to Pro Res via MPEG streamclip. The audio is 32-bit floating point . These are the files that need constant rendering.

I have other footage which were originally MXF which I also converted to ProRes via Brorsoft MXF Convertor. Here the audio is 16-bit. No need to render.

How can convert the MP4 files to 16-bit? Or can I change my sequence setting somehow so I don't need to render each time I would like to hear the 32-bit clips.

Thanks!