There was slated for production a few figures that would be able to talk and thusly had speakers in them. It appears to me that LJN or where ever this liono came from just used one of those molds. I had a pantho hard copy of this that was similar just not completely put together or sonic welded.
Hi Guys
Here is the inside of an hand-painted non-sonic welded Capt Cracker. I have had for about 5+ years.
https://lh4.googleusercontent.com/-5ihbpjOaobU/TgvPaCUYlBI/AAAAAAAACHI/hTiHSiTEYkY/s512/DSCF5386.JPG
Quite neat on the inside.
Going through some images on my computer and found these
http://i776.photobucket.com/albums/yy49/thunderianroyalguard/mummravariant1_zpsc099fe1d.jpg http://i776.photobucket.com/albums/yy49/thunderianroyalguard/mummravariant3_zps5c8088af.jpg http://i776.photobucket.com/albums/yy49/thunderianroyalguard/mummravariant2_zps98022e2d.jpg
Normal Mumm-Ra, but the hole is weird. Just like the Lion-O, the hole doesnt look like it’s been added later, but like it came from the factory this way. The Light-up connect is broken, which allows a peek inside. Any thoughts?
That is really very intriguing! By the looks of it, it doesn’t look like these holes were made by any of the previous owners. They look like factory made. My thoughts:
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As some of the other posters said, maybe they were for the “Talking Figures” which were never made. The holes would serve as an outlet for the sound. However, if this was the case, then why are the holes on the back in one figure and on the front in another figure? Besides, something like this should be in a prototype and not the factory produced toy. I doubt they would go through the trouble of creating a separate mold and then only making a handful of figures.
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Maybe these holes were for an unproduced accessory that would snap on the figure? Like an armor or back pack? Again, the above points make this also unlikely unless these were prototypes.
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My best guess is that these were factory defects. Maybe while pouring the plastic in the mold, some air bubbles formed which resulted in these holes when the plastic solidified. I’ve seen this in some action figures (can’t remember which, but not ThunderCats)
Whatever the reason, it’s great to see yet another variation from your incredible collection, my friend! 
I think that the holes are leftovers from the talking figures that were going to be released and then scraped but they had already made a few tests on actual molds and just released what parts they had left over at the end of the line. Basically LJN ran through there leftover parts bin at the end of the production line and used the leftovers to put together the last several production runs