This is an interesting question indeed and sort of similar to "are stereo speakers instruments"? Well, obviously they emit the organised sound closely matching that... of original instruments used in a recording.
I actually suspect that this thread was inspired by seeing some videos of music actually composed for speakers used as instruments (with interference applied to them or whatever) or the one with a laptop used with a chamber orchestra to "mix live" baroque pieces or something like that.
Well, I shall give a twist to the thread by saying this: I think the true instruments are acoustic instruments. The electronic ones are just artificially creating something which may match the original acoustic ones. I have no intention of igniting an endless "ping pong" of argumentative posts. Just use it as an example of a way of thinking, not necessarily right or wrong. However, I have come to some interesting points in this area and would be more than happy to see your comments on the following:
1. electronic pianos
They can also play the sounds of "violin" etc. are an obvious attempt at closely matching the "original" in one box and yet are no match for a separate instrument with its subtle individual colours and richness - so even though you can learn how to play piano using such devices or imitate the sound of symphonic violins, it still is no match for a Bosendorfer or a Stradivarius
2. things like theremin etc.
They can imitate eerie soprano-like sounds as well as cello-like etc. and still - do not fully reflect a real human voice or a real cello
3. acoustic vs. electro-acoustic
Come to think of it, many cultures around the world developed very similar instruments - regardless whether Chinese, Japanese, Native American, African etc. - there are many similar flute-like instruments, guitar-like instruments, harp-like etc. So I tend to believe that the instruments themselves are just physical representations of some higher order of more abstract and universal things like harmonies etc. However, these abstract things gain their perceivable shape and richness only when physical objects interact with the air whereas a stream of electrons coming into the equation will only give an additional twist or an approximation lacking depth and multidimensionality.
Just to give you an example for the last hypothesis: electric guitars are loved by metal fans for their ability to create powerful "heavy" sound - and yet, the cello section of an orchestra can give you something as heavy and intense, and yet a bit more complex and perhaps more pleasant to the ear. Similarly, guitars are loved for their "rough" sound but this is also something you typically get from a wheel fiddle (also called the hurdy gurdy):
http://en.wikipedia.org/wiki/Hurdy_gurdy
I generally believe the common misconception that electronically you can create more varied sounds is somehow incorrect - in the way that there are a lot of forgotten or only regionally known instruments (wheel fiddle, daf, cajon etc.) which can offer a lot in terms of variety.
And yet there is another twist to the story. Having said all that, I rarely listen to live instruments playing. So, I am listening to an approximation generated by means of the speakers or earbuds combined with the circuitry of a computer, home stereo system or portable media player. And therefore, the question could be: in our daily listening, do we actually listen to instruments?