After searching the forums, I do not see that anyone else has reported this yet (apologies if I missed it though or if you’re already aware of it). During the introduction of the Tau Cannon in Questionable Ethics – the scene where Freeman hears the unlucky guard and the scientist talking through the wall, then the guard accidentally overcharges the gun causing their demise - it seems the charge-up sound of the Tau Cannon is missing during this conversation. It was present in the 2012 free mod version, but it’s no longer in the retail version. I’m not sure if this is an accident or it was an intentional choice in order to overhear the dialogue better, but I’m assuming for now it’s a glitch.
Here are two youtube links I found for comparison. At first I thought maybe it was only me, that my sound file had been corrupted or something, but since it’s also missing in this youtube user’s playthrough, I guess that means the sound file was lost in the update.
2015 retail version: youtu.be/-RPhaQnno3A?t=684
2012 mod version: youtu.be/qE5o7vgnH4I?t=432
edit: If the youtube time index links I posted don’t work, go to 11:24 for the first vid and 7:12 for the second