:-? But suppose the subject of sound is plural? I admit that the singular is more common, when - as is often the case - 'Sounds perfect' is an elided from of 'That sounds perfect.' But if I'm inviting contributions to a picnic, and there's a range of suggestions: '[Those all] sound perfect.'
Of course, the 'Those all' is less likely to get elided (though it could be: 'I use IDE disks.'/ 'Still using those old things? Why not use SATA disks? They're quicker, quieter, and less prone to error.'/'Sound perfect'). But one can't just say "'Sound perfect' is wrong"; sometimes it's not.