unfortunately this is a bug that affects all goldsrc mods (even including the expansions) and there's no real workaround. the most you can do is play it thought an ambient_generic but that would require a lot of compression of the audio file
since it's a monster, it's gonna glitch out whenever it's inside something. however i don't see why you would need to use monster_furniture for this though if it's just a decoration, use a cycler_sprite