I hear ワンマン運転 as ワンマウンテン, with the ん of マン dissapearing, or perhaps being very soft.
Here's an example:http://youtu.be/DZnuH1XkE7s?t=49s
Another question shows that there can be something phonetically special about ん＋vowel words. This suggests the 'u' sound might be nasalized, but I don't hear the 'u' sound being different from a normal う in this example.
How do I pronounce ワンマン運転? Is this different from ワンマウンテン? Is there a rule that applies to some other words? Or is it a special case like 雰囲気?