assalamu alaykom the first word ends with a short "mu" sound

Assalamu alaykom it's religious expression

Both r right U can say also alsalam alikom

Assalam-o alaikum its due to the rules of naho in arabic that adds "dammah" on the 1st noun of a sentence called "mobtada'" in english the starting

Assalam alaikum السلام عليكم