On my N800 I seem to recall some bug that espeak wouldn't have sound if you just typed espeak "hello". You had to wind up creating a text file with the word hello in it and tell espeak to speak that. Then the word "hello" was spoken. This was a workaround but not a great solution as speaking from a text file seemed to have a large delay before starting to speak, which I attributed to having to open, parse and close the file rather than redirect from standard input.

Again this was on the N800, not sure if N900 version would have a similar bug or not.