Undesired Tremolo on playback (not on exported audio)

[Checkingscore]

Why are all my Sibelius files suddenly playing back
with a weird constant tremolo?

This is true of new and old files and only started today
(I've been a user for years.)

When I export audio, that goes away on the MP3.

Mac OS 12.7.2
Sibelius Ultimate

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When I try to attach a sibelius file and an MP3 file (both less than 1MB in size):

The sibelius file won't select
The MP3 file is grayed out.

Thanks for any tips.

[Checkingscore]

Actually, now, both the Sibelius and the MP3 file are grayed out, so I can't attach a sample (why is that? How do I fix it?)

(Below is a screenshot of the grayed out Sibelius file.)

[MikeLyons]

You can't attach sib files to this board directly. If you zip the file, it will attach. All files of any type must be less than 1M in size.