Try this:
Verify that you have sound enabled in The Matrix Online.
Check volume knob and wiring connections.
Verify that you have sound in other applications.
Reduce DirectSound Hardware Acceleration
To do so, follow these steps:
Click Start, and then click Run.
In the Open box, type dxdiag, and then click OK.
Click the Sound tab.
Under DirectX Features, move the Hardware Sound Acceleration Level slider all the way to the left (the No acceleration setting).
Click Exit, and then start the game
If you can now hear the sounds as expected, repeat these steps and increase the hardware sound acceleration level setting one notch, and then try again.
Update Sound Card Drivers
Contact your sound card manufacturer to obtain the latest version of the driver for your sound card.
Install the Latest Version of Microsoft DirectX
To download and install the latest version of DirectX, visit the following Microsoft Web site:
Microsoft Direct X download Hope that help