The Beolab 5000 has a unique design - and users need to know how to calibrate the output levels for the various sources, while also setting the channels right. I take it you don't have the manual?
Here's what you need to do.
Place the amplifier upright, on its left end. Put something underneath it if you want to avoid any scratches to its surface.
This gives you access to the underside. Now, it is quite stable in this position, but do be careful not to topple it.
You need a narrow regular screwdriver.
You'll see that you have a selection of sources available. See the photo below - the various pots are there to help you adjust the output level and balance of each source, and there's a special procedure. Note the TEST button on the front of the Beolab 5000 - you'll need to use that, as we proceed.
As different sources will have different output signal levels, this was how you set them so that each would appear to have a similar level when you went from one to another using the front buttons. Right now, the channel you are using for the CD is set very low, and we need to adjust that.
Here's what you do:
1. Push in the AUX button.
2. Play a mono source through your CD-player. A challenge to find these days, but you're certain to have one.
3. Set the VOLUME slider to 4.5
4. Set the BALANCE all the way to the LEFT.
5. Using the screwdriver, turn both AUX pots all the way down - both L and R - so that the volume is as low as possible.
6. Now turn the L pot up until you like the volume it's playing at (here some would actually compare the level to that output from the record player or tuner, switching between sources. But let's focus on getting a good CD-level now.)
7. Now move BALANCE to the center position - 0
8. Press the TEST button on the front panel, and hold it in - while turning the R pot until the sound becomes as WEAK as possible. (Not kidding you).
9. Let go of the TEST button. You will now hear a perfectly balanced output to both channels, at a level you like.
This was the world's most advanced amplifier when it was launched, and it shows.