fix: optimization: PVS-Studio warning V831

Replaced 'at()' method with 'operator[]' to improve performance.

The 'at()' method performs bounds checking, which can introduce overhead. Using 'operator[]' bypasses this check and can improve performance when you are certain that the index is within bounds.

Reported by: PVS-Studio
Signed-off-by: Vyacheslav Ivanov <islavaivanov76@gmail.com>

src/audio/audio.cpp

@@ -253,7 +253,7 @@ speakerid_t audio::play(
     if (!sound->variants.empty()) {
         size_t index = rand() % (sound->variants.size() + 1);
         if (index < sound->variants.size()) {
-            sound = sound->variants.at(index).get();
+            sound = sound->variants[index].get();
         }
     }
     auto speaker_ptr = sound->newInstance(priority, channel);