I personally can no longer abide a steam engine without sound. The chuffs, whistle and bell simply make the engines "come alive" in a way that silent running does not. Plus it makes it much easier to run the engines at realistic scale speeds when you can hear the chuffs.
Diesel sound is OK - I've converted my entire fleet - but mostly enjoyable when doing switching moves where you hear the prime mover throttle up and down (particularly that EMD 567 or Alco 251). But overall I find diesel sound less engaging than steam sound.
John C.