Does anybody what "Legni - Tr. sord." means ?

Something to do with wood instruments ???

I think that woodwind AND trumpet should keep quiet. It depends on the score - do some instruments share the same staff, so do they play exactly the same notes? And at this point (a quieter passage) only one of the instruments plays this staff?

Sounds VERY plausible!
As Jeeves and/or Wooster would have said: Rem acu tetigisti!

I had never heard of it either, but according to www.dolmetsch.com, it's a trill between two notes more than a tone apart.

This intrigued me, I really should not have ben here, and perhaps you, Rob, will wonder why I am.

An earlier comment is helpful. What instrument is playing to respond to the command? Who was the composer? When was the work written?

In a 20th century context and in the absence of any other context I would read it as:

Strings - put your mutes on and play tremelo with the wood not the hair of your bows!

But in an earlier century I would be inclined to agree with Rob. This is a passage for muted brass and woodwind (not muted as mutes are ineffectual on instrments with more than one exit).

An example of a double trill would be E and C trilling to/between F and D, on an instrument which is capable of playing two (or more) tones simultaneously.