Sounds are set up independently. If the speaker itself is working, then you need to go into "Settings" and check the sounds for ringtones, apps, notifications, etc., and make sure they aren't turned down or muted.
Next I would check that the sound you used for your ringtone wasn't deleted. If it was, either by you, but an update, or by an app you installed, that would also lead to no sound until you select another ringtone. In regard to Youtube videos, check the settings within the app itself. It is really easy to mute them or lower them without noticing.