There is a free online talking dictionary of English pronunciation. Just click over the pink words to hear them spoken at www.howjsay.com/index.php?word. Here is a pronunciation site recommended by Jelena http://www.pronuncian.com

I've realised English is fairly heavy with the ends of its words. Luis has given a fairly detailed explanation of what to listen for. The "b" sounds thicker, of course.

按照语言学的用语发音 ''b" 时 声带震动起来，而发音"p"声带不震动。 而且在浊音前面我们保持(?)元音的发音更长一点。 讲英语的人无意识地那么说。一般人没有意识到他那么说。

This is tough to explain in words! See if you can find recordings of a native speaker to listen to. You may want to do a web search to learn more about voiced and unvoiced consonants. (You can search for 'voice' and 'phonetics' together.) This difference will be similar to the difference between the consonant sounds at the end of the following words: hit hid hiss his pick pig