All was to no avail, till I remembered that I found it by searching for Florence Foster Jenkins (and not, as I thought, the Portsmouth Sinfonia).
So, from File audio - Brass Playing Mishaps (I'm not as 1337 as I used to be, so I didn't manage to convert the file - get the original for slightly less-bad quality):
"Ecco una serie di File audio "umoristici", rigorosamente anonimi, che circolano come allegati alle E-Mail o che si trovano su siti Internet.
Se avete un file che non è nella lista siete pregati di spedirmelo come allegato ad una E-Mail.
Tuba Mirum Blooper - Mozart
Mozart - Tuba mirum dal Requiem (Formato file: Real Audio)"
It's not as bad as I remember, but we take what we can get.
2024 May 27 update:
imeem no longer works, but luckily the original website still does.
I have converted the file to MP3 once again and am hosting it on 4shared (I have 2012 files on there so hopefully it stays up - as long as I don't miss the reminders to login or have my account wiped)
Mozart - Requiem - Tuba Mirum Blooper.mp3
Sadly, I was unable to convert such a short clip to video with Voice2V Express, so I can't put it on YouTube.
