I had the same first morning with my PT2, and the silence is part of why I wrote my own alarm app.
TTMM Alarms carries four tones of mine — not system sounds — normalised loud, because an alarm has to wake you rather than inform you. Sound is on by default, there are three volume steps, and the vibration keeps working even when you mute it. The alarm wakes the watch by itself with the app closed and rings full screen.
Whether any of my four is the bib-bib-bib-bib you mean I cannot promise. The trial is 48 hours, which is two mornings to judge it. And if none of them is that sound, say so here — that is the kind of thing I can act on.