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It sounds like the Vol Down button may be sticking on either the remote control or on the system itself.
Try taking the batteries out of the remote control first to check if it is causing the problem.
If the problem persists, then check that the volume button on the Beosystem control panel is releasing correctly and not sticking in.
Are there any other remotes that can also be used to control the system? If so, eliminate them too.
If not, then press and release the Vol Down button on the system several times to help unstick it - even though it is not pushed in, the electronic contacts behind but button may be shorted out and making contact.
If that doesn't work either, then the control panel will need to be opened to check those contacts and clean or adjust them.