As PhilipR said, the difference is small. It �s a matter of emphasis, as with the difference between simple tenses and continuous tenses.
"I was walking to work ... " focuses on the process.
"I walked to work." focuses on the result.
The same is true here, but to a smaller degree.
I think I can show you the small difference by expanding one of your examples.
"I saw him taking the envelope, and told him that he should have asked first." (This all happens at the same time)
"I saw him take the envelope. I hope it wasn �t your last one." (The action is completed, and the emphasis is on the result, not the process.)
"I heard the birds singing." In this, the focus is on the process over time.
"I heard the birds sing." In this, it is a simple statement about a past event.
The difference is rather small, but there is a difference.
Bruce
