Apparent contradictions in the biblical text are always difficult to deal with, usually because (1) we pick passages out of context and this isolation makes the passage look like a contradiction, (2) we treat scripture as if it is a modern-day historical text, or (3) we fail to understand what the author is trying to achieve through the way he has written the text.
Now, your passages from Acts are a great example of this. Since St Luke has written both of these passages as accounts of Paul’s encounter with Christ, and since Luke was a pretty smart guy and talented writer of biography and history, it seems very unlikely prima facie that he would make such a stupid mistake as to write an obvious contradiction.
So, let’s look at them.
 Now as he journeyed he approached Damascus, and suddenly a light from heaven flashed about him.
 And he fell to the ground and heard a voice saying to him, “Saul, Saul, why do you persecute me?”
 And he said, “Who are you, Lord?” And he said, “I am Jesus, whom you are persecuting;
 but rise and enter the city, and you will be told what you are to do.”
 The men who were traveling with him stood speechless, hearing the voice but seeing no one.
 Saul arose from the ground; and when his eyes were opened, he could see nothing; so they led him by the hand and brought him into Damascus.
 And for three days he was without sight, and neither ate nor drank.
" As I made my journey and drew near to Damascus, about noon a great light from heaven suddenly shone about me.
 And I fell to the ground and heard a voice saying to me,
Saul, Saul, why do you persecute me?'  And I answered,Who are you, Lord?’ And he said to me,
I am Jesus of Nazareth whom you are persecuting.' ** Now those who were with me saw the light but did not hear the voice of the one who was speaking to me.**  And I said,What shall I do, Lord?’ And the Lord said to me, `Rise, and go into Damascus, and there you will be told all that is appointed for you to do.’
Firstly, the contradiction - if there is one - here has to do with hearing rather than seeing. Notice the first passage says that the men saw no-one and the second passage says they saw the light. These can be seen as essentially equivalent, since in saying they saw the light, Luke is still emphasising that they saw no person (who was speaking).
So, how could they have heard the voice in one instance and not heard the voice in the other? To respond to this, we need to look more closely as the Greek text, and not at the word “voice” but the word ἀκού, which can mean both “hear” and “understand”, depending on the context. It can genuinely be translated either way, and I think to get a good idea of what Luke is talking about, we need to go back to his Gospel: Luke 8:10 -
“To you it has been given to know the secrets of the kingdom of God; but for others they are in parables, so that seeing they may not see, and hearing they may not understand.”
Jesus is hear explaining why he speaks in parables, quoting from the prophet Isaiah. Notice he points out that people see but don’t see, hear but don’t understand.
I suspect Luke is re-working Paul’s story here (or perhaps even Paul himself is) to make this a living parable. Unlike Paul, those with him see and hear but don’t understand; just like Paul, they witness the revelation of Christ - they see the light but don’t see, they hear the voice but don’t understand. I think this is what is going on in this passage.
Again, notice how biblical interpretation is not simply a basic matter of taking two passages out of context and saying that they contradiction. Luke, and the other evangelists, were very clever and intelligent crafters of their Gospels (they were inspired by the Holy Spirit after all!) and we need to treat their work with such respect.
Now, with regards to Paul’s Letter to the Galatians, I’m not sure how that quote shows Paul to be a “liar and hypocrite”. Could you elaborate?