There is no set limit on how much a Folder can hold, or the type
of files it can hold.
Your music files are likely corrupted by 3rd party programs, or
during extraction from compressed folders, or were simply
damaged/corrupted by 3rd party player apps.
Files can also become corrupted or damaged because you used
characters like these in naming your files; more than one full stop
(xx.xxx.xx), use of commas (xxx,xxx.xxx), use of reserved
characters (/ \ {} [] | * % $ # @ ! < >)
Some of these reserved characters or symbols cannot be used even if
you try, as they're reseved for programming languages.
Using more than one (.) can render an executable file corrupt or
orphaned. Sometime, files with too long a name can become truncated
and hence become disassociated with the programs that runs
it.