Isteach means "in" so tá fáilte romhat isteach is "welcome in", though you might be more likely to say "Please do come in" or such in English, the clear implication is that you have stepped into a specific room or building, rather than a general welcome on arrival at a place.
In DL Irish, there is always a blue "sound" icon at the top of the discussion page, which quite freqently (one out of three to four?) gives a sound sample (as is on this page as well).
Now the quality of the speaker is considered extremely poor by the experts, but I still appreciate a poor indication more than no indication at all. From where I come, "romhat" would be pronounced (and silently read) as the english "rum hat", which definitely is much worse, than the poor indication.
It depends on the speaker's dialect. In Donegal, for example, a broad bh or mh is usually pronounced like w, so 'an bhfuil' sounds a lot like 'uh wil.' As paddyobrien notes above, though, they are often pronounced as v at the end of a word. Bh and mh are usually pronounced v all the time.
Pronunciation rules differ from dialect to dialect, so it's usually impossible to give a single simple rule. The pronunciation database at teanglann is a great help, but be aware that there are pronunciation and usage differences within the dialects; there isn't just ONE Munster pronunciation, for example.