In 1889 the language was two years old, and its speakers were few and far apart, as the poem itself says. This is Zamenhof writing to them: where are you, what are you doing, has your courage blown away like a breath of wind? And then the answer he needed them to hear: ten calls will be laughed at, and the eleventh will take root.
The recording is a piece of history in its own right. In 1926 the Esperanto Institute in England brought out the first Esperanto music records, and this was one of the three titles, alongside La Espero and La Tagiĝo: sung by Maurits Boeren, an Esperanto teacher from Antwerp, in a London studio that July. It sounds like a 1926 acoustic disc, because it is one.
The grammar doing the work is the -u ending. , brave , floru: not commands but wishes, the "let us" and "may it" of the volitive mood, which is the same -u as the imperative and . One ending, three English constructions. Conditional and commands has the whole of it.
Watch, too, how much the poem builds with -: , , o, o, o, a, malfortiĝante. Seven opposites, one prefix.