There are both internal and external speaker output settings in the sound preference pane. In order to adjust (or turn off) the volume of your Mac's internal speakers, nothing must be plugged into the headphone jack. Keep in mind that this setting completely controls the volume of your internal Mac speakers, so if you mute the sound, and have no external speakers, you will get no sound at all from your Mac.
To adjust the volume on your Mac's internal speakers:
- Unplug speakers or headphones from the headphone jack.
- Go to System Prefs > Sound, then click on the Output tab.
- You will then see Internal Speakers (Type built-in) highlighted in the window.
- Adjust the output volume (which also controls the chime volume) in the slider at the bottom of the window. Remember if you mute it there will be no internal speaker sound.
- Plug in speakers or headphones to the headphone jack.
- Go to System Prefs > Sound >, then click on the Output tab.
- You will then see Headphones (Type built-in) highlighted in the window.
- Adjust the output volume (which controls the chime volume in the external speakers) with the slider at the bottom of the window. Remember if you mute it there will be no external speaker sound.
[kirkmc adds: Depending on your setup, this can be a bit more complicated. For example, my main Mac is a Mac mini connected to a DAC via a Toslink cable, which uses the headphone jack. When you use the digital output like this, you cannot control its volume. I also have a Cinema Display, connected via Thunderbolt, and it has internal speakers; so I can set the volume in those. This shows in the Sound preference pane as Display Audio.
If you have a Mac Pro, you have a tiny - and tinny - internal speaker. If you have an iMac, you have the same type of speakers as I have in my Cinema Display. And if you have a laptop, you have one or two small speakers.]

