The PlainTalk sound input mini-jack is a standard 3.5 mm diameter. The symptoms you describe occur if you use 1/8-in connectors instead of 3.5 mm connectors.
The differences:
- 1/8-in. plug is .125-in. in diameter
- 3.5 mm jack is .139-in. (0.014-in. bigger)
When a 1/8-in. plug is used, it is prone to cutting out and having excessive noise on the audio line. To prevent this, you should use a 3.5 mm connector instead of 1/8-in.