In fact, i've already known how 2 say. But i've just got an audio tape from beijing daxue in which they say it like sound"v"+yunmu "en". That makes me confused. When i say "wun" i mean "un" ( un=uen).

No we never pronounce like "wun" for "ask" in standard pronunciation, they sound indeed alike though. We always say "wen". Exactly.

好好练习，即使是中国人，发音也比一定掌握得标准。

The mouth when nasal pronunciation, do not use nasal pronunciation, Imagine your nose stuffy went.Pronunciation to brief.

prounce "wen"