A sleeve is made available for hearing aids, hearing aids provided therewith, and parts. The sleeve is in the form of a flexible tube or sack and is placed over the hearing aid, (in-the-ear unit) which is inserted in the auditory channel or that part of the hearing aid (behind-the-ear units) which is inserted into the auditory channel. The diameter of the sleeve, which is in the form of a flexible tube, is variable and is thus moulded against the auditory channel wall. The sleeve, which is in the form of a flexible tube, is thus used both for locking the hearing aid in the auditory channel and for protection of the hearing aid against cerumen etc. Furthermore, a hearing test device is made available, using which, in particular, the abovementioned hearing aids can be individually adapted to a wearer. Using the hearing test device, various sounds and sound events can be played back or can be made audible. The test person varies the operating parameters of the respective hearing aid until an optimum sound impression is produced.