It's quite difficult to find much technical information on how the Currah μSpeech ("Microspeech") works. The manual (section 6) does delve into it on a functional level, and this link is an excellent write-up of memory addresses where the device functions from.

Why this is relevant is because; the Spectrum Computing forums have cleverly found out that the Goldfish minigame is probably activated via this routine.

Note; obviously doing this crosses over the 65535 boundary. On a "normal" Spectrum the ROM is not writable at all (hence ROM, it's Read-Only Memory), however with a Currah μSpeech unit attached, 0000 is part of the device RAM - and hence is then writable (and will write 0 up to 0095).

As for the "why does this activate the minigame?" that is still somewhat still waiting to be discovered.