If by “Sung Low Mass”, you mean a Low Mass with Hymns, in which the celebrant sings nothing, then no, the servers sing nothing.
If you mean a Missa Cantata, which is a Sung Mass (with priest but no deacon or subdeacon) but follows more closely the rubrics of a Low Mass than a Solemn High Mass, then the servers’ primary obligation is to make the responses to the priest. Afterwards, the servers may join in the singing if appropriate. For example, the servers recite the responses in the Kyrie, but once the recited Kyrie is over, they may join in singing the remainder of the sung Kyrie with the congregation.
