Audio Problem

[Johnny99]

Have a '98 Avensis CDX & to date have never had any problems with the factory fitted radio. However yesterday when driving along with the radio on the volume suddenly went right down. I tried different radio stations and tried a CD incase it was the radio reception but all with the same result very little to no volume. As i was driving along i realised that what was happening was when i was accelerating the volume would go low but when i braked or was stopped the volume would return to normal level. It could be a fuse but i can't see how that would only affect the volume when the accelerator was pressed and not when the brake was applied. Anyone have any ideas what would cause such a strange problem?