No, there isn’t.  You need to switch CEC off on the TV and switch the speakers manually. 

Thanks

But does it appear that is something quite reasonable to have as a feature?

That would require the Beam 2 to turn off CEC on the TV, and CEC (Consumer Electronics Control) is the protocol used to control electronics from another device.  Once CEC is off, the Beam 2 would have no way of turning it back on.  Bottom line is, CEC expects a soundbar to be the source of sound, and makes it difficult to switch back to TV speakers, something I agree with.  I’ve never willingly reverted back to TV speakers in 20 years of owning home theater components and don’t know why anyone would.  But I understand it is a personal choice.