Asterisk sounds are rooted in the astvarlibdir (as defined in /etc/asterisk/asterisk.conf file) /sounds directory, then as documented in :-
https://wiki.asterisk.org/wiki/display/AST/Sound+Prompt+Searching+based+on+Channel+Language
the default language is en_US and is hardcoded in the asterisk source in many places. So my guess is that it will probably fail at various places if /var/lib/asterisk/sounds/en at least is not there whether the file exists in that directory or not, and further cannot play any file asked for but not there in the descending tree as the link describes. If you build asterisk with make menuconfig the default would be to create only the basic core sounds in /en , most distros will also provide the en core sounds plus the extra sound packages in en also.