Each country of the Union may impose for itself reservations and conditions on the exclusive right granted to the author of a music work and to the author of any words, the recording of which together with the music work has already been authorised by the latter, to authorise the sound recording of that musical work, together with such words, if any; but all such reservations and conditions shall apply only in the countries which have imposed them and shall not, in any circumstances, be prejudicial to the rights of these authors to obtain equitable remuneration which, in the absence of agreement, shall be fixed by competent authority.