You may want to review this Apple doc regarding supported file types, codecs and their dependance on which version of Mac OS your clients are running.

The only common audio format between PD's MP4 output and iMovie is AAC, but if you're not happy with that you could produce the MP4 as usual and then produce an audio-only .WAV clip at whatever settings you find viable. Since you're producing from the same project both clips should be perfectly synchronized.

I've never used iMovie but I assume it can import video and audio tracks separately and mix them into a final product. If that's the case, then have you clients mute the embedded AAC audio and only use the separate .WAV track.

Thank you- I'll give your approach a try. I just don't understand why PD19 doesn't produce acceptable audio when rendering into MP4.

I don't know what your produced audio sounds like or why you consider it unacceptable. I don't recall having issues with AAC quality when producing to MP4, but it's possible that something about your source clip's audio might cause a problem when PD transcodes it.

You can create a custom produce profile with a higher audio compression rate of 256kbps or 384kbps and see if that sounds better.

I can confirm that a mp4 video with LPCM audio is silent on the iPad with iMovie. However it should be okay on the Mac with the iMovie using the latest 10.2.5 version. See this article: https://www.sony.com/electronics/support/articles/00065722 .