How do I turn off sound notifications

Click “eM Client / Preferences (Settings) / General / Notifications”. Then click the speaker icon next to “Incoming” etc to disable sounds as per the eM Client Documentation link below.

https://www.emclient.com/webdocumentation/en/9.2/emclient/default.htm#Settings/Notifications.htm?Highlight=Notifications

(eM Client windows example below)