This is correct if your only goal was to secure a conviction, but you are forgetting that when witness tampering is occurring, there is already a judge who is v

This is correct if your only goal was to secure a conviction, but you are forgetting that when witness tampering is occurring, there is already a judge who is very willing to hear evidence about it. The integrity of their court is at stake. They absolutely do not require an audio recording to make any number of potent orders to protect you. The indirectness of the threat and its unsuitability to curing by judicial power is more of a problem than the lack of audio recording. Your best bet if you care about delivering the evidence is probably to leverage the power of the judge, ventilate it and get the people threatening you to retreat. If you do that, you transform a threat behind closed doors (not to do something) into a retaliation for something real that you did (telling the court about tampering) that you can tell your tenure committee about later. In sum, open justice is designed to expose these things, it is far and away the best forum for it, so better to use it while you can.