AISO4ALL 'takes over' the soundcard; system/program sounds cannot be heard while it is in use. But if you have other programs which are using the soundcard prior to opening FL Studio, AISO4ALL will not start properly.
SOLUTION: Close any applications such as WindowsMediaPlayer, iTunes or Firefox which might be using the soundcard when you open FLS. Go into Options>Audio Settings, switch to "Primary Sound Driver," then back to AISO4ALL. You should be able to hear FL Studio's sounds just fine now.
If not, close FL Studio and reopen it. You should hear the 'blip' startup sound now.
Yep, but in his case, it was just the audio settings of Windows (volume). You're correct: ASIO4All 'takes over' the soundcard, but it can't bypass all the settings, it can be seen as some kind of.. awesome wrapper. When the volume of the soundcard is set low, this can also apply to the ASIO drivers which results in.. well, a low volume.
The procedure described by you is a very good workaround when there is no sound at all.
It also often helps to change the sampling rate of the soundcard (down to 48 khz from 192 khz, for example) if ASIO shows strange errors and does not play any sound.