The idea is to replace all something(a|b|c|...)
with lines somethinga
somethingb
etc.
awk seems to be suited here more than sed:
awk 'BEGIN { FS="[(|)]" ; OFS=""} /\(.*\)/ { for (field=2;field<NF;field++) {print $1,$field ;} ; next ; }1'
This will do what you wanted