Here's a picture of one:
Is there a simple but clear way of referring to it?
I'll post an answer (partly because of Mari-Lou A's prompt!) as there is an important point of English usage that is involved.
Certainly, only furniture experts could reasonably be expected to object if the bit of wood was referred to as a 'desk shelf'. Using desk as a 'noun-doing-an-adjective's-job' (I'm in between terminologies on these) is quite permissible and idiomatic - it can mean 'part of', 'belonging to' - even 'somehow related to' (a desk / desks) in this sense. We won't go deep into 'things related to desk-related things' like desk officer! However, a decent dictionary will list accepted compounds such as
But I'm fairly sure 'desk shelf' will not be listed. Again, 'white van man' may well have a separate entry, but not 'purple van man'.
So, using the term 'desk shelf' here is probably as simple, clear and sensible as it gets (unless you cleverly add a highlighted picture) - but it's not an agreed and well-defined term. Without the picture, we couldn't distinguish between a one-shelf and a two-shelf desk, an above-desktop and a below-desktop shelf...
And even where 'accepted' terms are recognised by relevant authorities (Sotheby's? Miller's?) there are often international, regional, and organisational disagreements.
Shelf is the word I always use. You don't necessarily have to put speakers on it, either. That's your choice.
EDIT: Sorry. Answer was a little too flippant. How would you differentiate a normal shelf from that of your desk? Could we say: The shelf above my desk? No... misleading.
In Italian it would be "piano" but again that's open to different interpretations. I'll leave my answer here to bump up your question, @janochen.
Found it! "Desk shelf" or computer desk hutch
I am an interior designer and would refer to this as a gallery shelf. This would be in the context that we are referring to a piece of furniture. There is a plethora of vocabulary to describe furniture, particularly antiques, which if not being taken in context would not guarantee a good result on google (unless a retailer had optimised that term).