There should be some feedback from the request to synthesize transcription preview on the EditLexicon special page.
When successful it takes a second or two to receive the response. The user should be able to tell when it is done and ready to listen to.
If there is an error from Speechoid, this should be displayed somehow, e.g. in a message dialogue. The simplest way would be a generic "something went wrong message", but these are seldom useful. It should at least include the Speechoid error, which looks like this:
ERROR: failed mapping transcription : found unknown phonemes in transcription /a:.pa/: [:]