An in-the-ear audio device includes a custom made shell part which fits into the ear canal of a user and a pre-fabricated module part, the custom made part having a cavity with an opening for receiving the module. The cavity is partially bounded by two opposed, substantially parallel internal walls of the shell part which are spaced a distance which will allow two external, substantially parallel sides of the module to slide into the space between the two walls. Recesses and projections which extend in an arc on the walls and on the sides are provided such that the module may be inserted into the cavity between the two walls by a pivotal movement, with the projections at each side sliding in respective recesses.