The present invention relates to mounting of a sound tube inside a shell of a hearing aid. The invention provides a sound tube adaptor, a tool for insertion of the adaptor and a method for attaching a sound tube to a shell, the sound tube adaptor facilitating attachment of a sound tube to a shell, having a first end for attachment to one end of a sound tube and a second end that is adapted to be received inside the shell in a hole in the shell in such a way that the adaptor and the sound tube extend inside the shell from the hole towards the entrance of an ear canal of a user when the shell is positioned in the ear canal.