Tente isto:
sort -t "(" -k2,2 -n word.txt
Saída
INSERT INTO area_names VALUES(12,'NewYork'); INSERT INTO area_names VALUES(124,'NewYork'); INSERT INTO area_names VALUES(125,'NewYork');
-k POS1[,POS2]
Specify a sort field that consists of the part of the line between
POS1 and POS2 (or the end of the line, if POS2 is omitted),
_inclusive_.
Each POS has the form 'F[.C][OPTS]', where F is the number of the
field to use, and C is the number of the first character from the
beginning of the field. Fields and character positions are
numbered starting with 1; a character position of zero in POS2
indicates the field's last character. If '.C' is omitted from
POS1, it defaults to 1 (the beginning of the field); if omitted
from POS2, it defaults to 0 (the end of the field). OPTS are
ordering options, allowing individual keys to be sorted according
to different rules; see below for details. Keys can span multiple
fields.
Example: To sort on the second field, use '--key=2,2' ('-k 2,2').