i have the same problem in languae French
i make chown www-data:www-data gsm*
I think you've got an issue with the command. Shouldn't it be:
chown asterisk:asterisk *.gsmor with complete path:
chown asterisk:asterisk /usr/share/asterisk/sounds/pluto/*.gsm
Can you else give us an output of following command?
ls -l /usr/share/asterisk/sounds/pluto