You all know how wary I am of anything that requires me to interpret sounds. However since this new proposal has the blessing of the Forum, floccan and the spirits themselves I'll give it a go.
What about 7pm GMT this evening (Monday)?
It would be far, far better if flaccon can provide the third. You, Alderbank should only observe, facilitate, and monitor. Flaccon needs an ally she has pre-screened and is confident in the ally's ability.
Here's a simple explanation of how I'd conceive the test be run (with three):
First, Alderbank selects a card and allows flaccon to view it. Ideally, Alderbank does not view the card, himself. For each card in turn, flaccon asks the spirits to identify it, then she listens to the recording playback to hear the card correctly identified. If this fails, flaccon will again ask then listen. She will not go on to the next card until she has a useable recording. Flaccon may request a substitute card be selected, but the total under of ask then listen attempts should be limited. Let's say 4 tries in total for the original and any substitute cards before declaring the test a failure.
ETA: Three cards + recordings are needed.
After each recording is deemed acceptable, Alderbank will seal the card in a numbered envelope and set it one side. The recording files should be named in some generic way that includes the card number (something like CARD1.WAV, CARD2.WAV, and CARD3.WAV would be good) and placed in a directory with no other files on flaccon's computer.
The computer should be left set up so the individual recordings can be easily played for flaccon's friend during the next part. Alderbank look over the computer screen and the general area around it for any (presumably unintended) clues to the cards' identities.
Flaccon leaves and the friend is brought in.
Alderbank will then replay each of the recordings for the friend, one at a time, and allow the friend to name the corresponding card from the recording. Let's put a 20 minute time limit on listening and relistening before abandoning that card.
The first card will be used for an open test. Alderbank will open the first envelope and show the card to the friend (ideally without viewing the card himself). The open test will give the friend the opportunity to become comfortable with the environment and that things working.
For the other two tests, Alderbank will write the card name provided by the friend on the corresponding envelope.
Once all three cards have been handled, flaccon will be brought back in and Alderbank will open the envelopes and permit both flaccon and the friend to compare the cards to what the friend heard on the recordings.