If you have already tried all the tips to try and regain sound ingame when it is silent (i.e. from LDSO and forums etc.), try the following which cured it for me:
If there is no speech/sound effects, go to windows control panel and select "Sounds and Audio Devices", then go to "Hardware" tab. From list, select "Audio Codecs" then properties, this then shows the list of installed codecs. The top one is the important one - Windows Media Audio! Select it and make sure it is enabled. If it isn't that's what's causing the lack of sound as the game uses this codec for the ingame audio(?). I don't know how or why it got disabled though.
Once I enabled it, hey presto! The audio was back along with normal cutscenes. Phew, well it took ages for me to work it out but at least I won't have to go through that again!