No Sound after External Monitor Installed

[jkl]

Windows XP; Thinkpad t40 laptop (being used as "desktop"); Gateway FHD2400
Monitor.

Monitor will display video, etc.; but no sound accompanies the video.

Also, when running applications with sound effects, no sound.

Any ideas why this would be happening and how it might be fixed?

Thanks in advance folks. And thanks again to Mark Ferguson and Shenan
Stanley for responses earlier today on separate issue.

 

[smlunatick]

Windows XP; Thinkpad t40 laptop (being used as "desktop"); Gateway FHD2400
Monitor.

Monitor will display video, etc.; but no sound accompanies the video.

Also, when running applications with sound effects, no sound.

Any ideas why this would be happening and how it might be fixed?

Thanks in advance folks.  And thanks again to Mark Ferguson and Shenan
Stanley for responses earlier today on separate issue.

How did you "activate" the external monitor? If you pressed a
combination of keys, you might have "turned off" the sound card.