Thanks for letting me know the problem with multiple dots in filenames.
I would solve it this way:
To change only the listed extensions of selected filenames to lowercase:
Action "Replace with regular expression"
Field: _FILENAME
Regular expression: ^(.*)(\.(mp3|m4a|flac|mp4|aac))$
Replace matches with: $1$lower($2)
(All not listed extensions will be ignored - you can adjust the list as you like)
To change ALL extensions in the selected files to lowercase:
Field: _FILENAME
Regular expression: ^(.*)(\.(.*))$
Replace matches with: $1$lower($2)
You could also replace the existing extension for all selected files to a fixed new extension
like .aBC with
Replace matches with: $1.aBC
To change only the filename in all selected files to lowercase, but let the extension untouched:
Field: _FILENAME
Regular expression: ^(.*)(\.(.*))$
Replace matches with: $lower($1)$2
To change the entire filename - including the extension - for all selected files to lowercase:
Field: _FILENAME
Regular expression: (.*)
Replace matches with: $lower($1)
If I would like to rename files individually per track as you suggest with your MTA's, I would use a specialized File Rename software with all the whistle and bells.