In standard Japanese, the sounds い /-i-/ and う /-u-/ usually become voiceless (i.e., whispered or de-voiced) if they appear between voiceless consonants /-k-, -s-, -sh-, -t-, -ts-, -p- or -h-/ or at the end of a word.
If the sentence ends with ...です (copula ending) or ...ます (polite verb ending), the final -u- sound is also de-voiced. If the sentence does not end with these words, de-voicing of the sentence-final /-i-/ and /-u-/ sounds may or may not occur. Compare the following:
Literal pronunciation
Natural pronunciation
history
れ き し
→
or
munch
ぱ く つ く
→
or
junk
が ら く た
→
I am Brown.
ブラウンです。
→
De-voicing phenomena are much more complex than what's described above, and there are many regional, phonological and situational factors to consider. We will not go into the details here. Listen to the native speakers' pronunciation carefully and learn by examples.
2: Intonation
Japanese is not a tonal language like Chinese and sounds relatively flat, but many homonyms (different words that look the same when written in ひらがな) are pronounced with a different intonation so they can be distinguished.
Since ひらがな does not differentiate intonation
patterns, the only way to learn the differences is by listening
carefully to the pronunciation of native speakers of standard Japanese. In the following,
the first きた is pronounced as the verb came and the
second きた is pronounced as the verb put on [clothes].
When syllables turn voiceless (き part), the differences become rather subtle. Click on the words below to hear the difference.
came
put on [clothes]
FYI: There are some differences in intonation patterns among various dialects in Japanese. The standard Japanese is spoken primarily by people who live in Tokyo or those who have learned to speak in the standard Japanese through broadcasting media.
or 
Compare the following: 
![Intonation contour for kita ("put on [clothes]")](../../../../images/text/puton.gif){kind=small}