The headword is the accusative singular of the feminine noun sunauli/a. See also xi 116, xi 117. Given the glossing this is evidently LSJ's sunauli/a (A), connected with the aulos "pipe"; LSJ note instances where it bears the precise meaning "concert of lyre and [pipe]", alongside others where it has the more general sense(s) translated above.
The lexica vary between accusative case and nominative; references at Photius sigma740 Theodoridis.
A scholion on Lucian (265.20 Rabe) distinguishes the two senses of sunauli/a: cohabitation (from au)lh/) or musical duet. Moeris the Atticist lists sunauli/an as Attic and sumfwni/an as common Greek.