The spelling of "Isaac" in 7.9 and 16 is unusual, יִשְׂחָק yischaq rather than יִצְחָק yitschaq.
At v.16 the LXX has "Jacob" but at v.9 it translates γέλωτος (laughter) this would suggest the yodh was either ignored or missing.
"Isaac" is only spelled with a sin here and in Jer 33:26 and Ps 105:9 (=1 Chron 16:6). However the verb, "laugh", though most often spelled with a tsadeh (צחק) in the Torah usually has a sin (צחק) elsewhere.
The Damascus Document (from Qumran) also once apparently seems to spell Isaac with a sin (like in Amos).
There are differing explanations for the existence of the two
spellings for the verb, to do with cognates in Western Semitic
langauges, but I don't find them helpful here. In contemporary
speech differing dialects do not pronounce words in identical
ways (think of the differences between American and English usage).
It is at least possible that there were dialectal differences
in the text as well as change and standardisation (particularly
of the patriarch's name) over time before the fixing of the text.
This page is part of the Hypertext Bible Commentary - Amos,
© Tim Bulkeley, 1996-2005, Tim Bulkeley. All rights reserved.