Is it an error (red) or a warning (blue)? I suppose it’s a warning as errors prevent you from publishing. Most of the warnings Lectora displays can be ignored. I don’t know what’s responsible for this warning, but it seems to be irrelevant unless you wanted to use the course on mobile devices. What types of audio / video files do you use?
In the end there’s no difference between courses for mobile and desktop. Responsive Course Design allows you to rearrange the pages for smaller screens or portrait dimensions, but it doesn’t make the course “mobile”. It would use tablet landscape or phone landscape on a very small desktop screen / laptop as well.
Seamless play is not related to mobile devices. It creates smooth transitions when navigating between pages. Many threads in the community may give the impression that more or less any issue in a course may be solved by deactivating, but it shouldn’t make a difference here.
I’ve checked the Information Center, all it says to this is “Some warning messages (displayed in blue) may never be resolved. These messages will not impede the publishing process. “