For me, "solid" here would have the sense of "well-written", "very competent". It's good work, as opposed to poor work done very carelessly. I leave it to you French-speakers to decide on the best translation.
Sorry, Snarkhunter, but I wouldn't really agree with this observation. For me, a "solid" work of music basically means "well-written", nothing slipshod about it, regardless of the purpose for which it's written. If I wanted to say that a work of music was "particulièrement adaptée à la circonstance", I'd have to think of something else.