A helicopter gas turbine engine having a combustion chamber, at least one turbine arranged downstream from the combustion chamber to receive combustion gases emitted by the latter and a nozzle is disclosed. The nozzle has one part forming a diffuser, connected downstream from the turbine and an ejector which has an upstream part surrounding the downstream end of the diffuser and defining therewith an outlet passage for an engine compartment cooling air secondary flow, the ejector running in the downstream direction beyond the downstream end of the diffuser. The ejector has a wall formed at least partially by a sound attenuator calculated to attenuate sound frequencies generated by the rotation of the or each turbine and/or by the combustion chamber.