Playback doesn’t replicate for sounds in SoundService to begin with. Setting TimePosition on a SoundService Sound will not replicate to the client. Do this all from a LocalScript instead.
Just because it worked for another property, doesn’t mean it’s applicable to TimePosition. TimePosition does not replicate across the client/server boundary and SoundService has esoteric playback replication rules, such as a server’s playback not replicating to the client as I just said.
If you just play the sound and set the TimePosition from a LocalScript then you’ll be fine. Additionally, canonically, sound playback should be controlled completely by the client and not the server. The server doesn’t have a concept of sound.