IBM 6266 User Manual page 30

• You can also plug headphones into the line-out jack on the back of the system
unit. This is the same jack to which the external speakers connect. You must
disconnect your speakers to use this jack with your headphones.
If you plug your headphones into the line-out jack on the back of the system
unit, the headphones will pick up all sounds generated from your computer and
any external devices connected to the computer, such as a MIDI (musical)
keyboard. The headphones will also pick up sound coming from a CD in the
CD-ROM drive.
Note:
2-8 IBM Personal Computer User Guide
When your headphones are plugged into the line-out
jack on the back of the system unit, you must control
the headphone volume through the volume-control
program on your computer. You cannot control the
headphone volume from the volume control on the
CD-ROM drive.