I had the exact same problem, and it pissed me off to no end. but I have a fix. I use S voice alot to automatically schedule appointments in the Splanner, and I ran into an issue when I double click on the home key, svoice would launch, but I would manually have to manaully turn on the "talk back button" (to the left of the mic) and push on the "mic" button, instead of all of this happening automattically like it used to do. And even worese, after clicking both buttones and after saying "create appointment factory" it would respond correctly by saying what time, but the chime would not sound indicating for me to speak the time I wanted to schedule the appointment. (like it used to do). This drove me nuts!
What I did to fix it, was uninstall a recently installed app, (in my case it was floating notifications), and then I went into the svoice settings and turned off "wake up command" went back to the home screen, opened svoice again, and then turned "wake up command" back on. Now it works like a charm again!
Please note, that I dont know for sure if the floating notifications app is what caused the problem, but this is what I did to get it back working again, you may have totally different apps, or totally different circumstances. What I would recommend, is ask yourself what apps have you installed recently or what settings have you changed recently since svoice was working correctly last. and then go from there. Hopefully this helps somebody.