Sound recording:

A disc, tape, film, cylinder, or wire on which sound vibrations have been registered so that the sound may be reproduced, or paper rolls on which the notes of a musical composition are represented by perforations in the paper and from which sound can be mechanically produced.

In Leader/06:

For a musical sound recording, the code j is used.

For non-musical sound recordings (for example, of speech, bird calls, sound effects), the code i (letter "eye") is used.

In 007/00 Category of material:

Code s indicates that the item is a sound recording.