You might find that things are better in PrPro, if you use SoundBooth, Audition or even the free audio editor, Audacity, to convert your mixed sources to PCM/WAV @ 48KHz16-bit.
PrPro can usually handle many mixed Audio sources, but can sometimes stumble. With MPEG Audio, one of those stumbles is often OOS.
You can certainly try those sources, and if you do have issues, go to Plan B (outlined above).
Exactly the answer I wanted!
As you know, I'm new to Premiere, but I want to try to do things the "best" possible way each time I take a new step.
Actually I just went back to the download source because I remembered that they offered WAV @ 48KHz16-bit or mp3.
I chose mp3 yesterday because it was about 1/4 of the size, but will download the wav files instead.
One of the biggest quandries with MP3's, is that many work just fine, but then some do not. More than a few users have had no issues, while it seems that others always hit problems. I have had some issues, so just adopted a workflow of converting every one, except for tiny SFX files, that do always seem to work. Possibly, my luck with those is that the collections that I have bought over the years, were done well?
In the end, I find that feeding PrPro what it likes best, always works. When going to Encore, I would triple-down on that suggestion, and feed Encore the very few formats/CODEC's, that it is designed to handle. I remove any sort of conversion, or editing, from Encore, and ONLY Import totally finished Assets, of a narrowly defined set of formats/CODEC's, regardless of what it can normally Import. Limiting things to just the ultimate formats/CODEC's has yielded 100% success for me.
Good luck, and do not hesitate to try things out. Armed with the info on what is the ultimate, will allow you to fall back to those specs., should things not work perfectly.