Thank you Raif

You advice was very useful. Here is how I proceeded;

• To download rootsh from Maemo repositories
• To open Xterminal and to write the follow;

~ $ sudo gainroot
/home/user # cd MyDocs
/home/user/MyDocs # ls -a
/home/user/MyDocs # cd .sounds
/home/user/MyDocs # rm -r directory
/home/user/MyDocs # exit
~ $ exit

Where directory was the name of the folder containing read-only files.

Of course the result was that read-only audio files were removed successfully!


Lessons learned;

• Definitely, you must download rootsh in order to gain "full access/privileges" (it is how I understood in my newbie level) to modify/erase files on system. Otherwise if you try just using simple Xterminal privileges, you certainly will find same files but you won't be able to delete them.
• I spent some time searching the folder containing the problematic audio files. It was hidden denoted by ".folder", so make sure to include the "-a" in your search to list hidden files and directories (I know, i know... this is a super basic recommendation)
• Ask always for help, some issues are really easy to solve, so don't act impulsively... some friends recommended me to reset/reimage/reflash fully the device... other people mentioned that my 36GB card was damaged and it shall be replaced... wtf?

Thank you for reading!