refactor(docs): fix Audio module documentation links and add missing getter docs

docs/api/audio/audio-system/audio-system.md

@@ -54,7 +54,7 @@ AudioSystem 是 XCEngine 音频模块的核心单例类，负责管理整个音
 
 ```cpp
 #include <XCEngine/Audio/AudioSystem.h>
-#include <XCEngine/Audio/WASAPI/WASAPIBackend.h>
+#include <XCEngine/Audio/WindowsAudioBackend.h>
 
 using namespace XCEngine::Audio;
 
@@ -65,7 +65,7 @@ void SetupAudio() {
     config.bitsPerSample = 16;
     config.speakerMode = SpeakerMode::Stereo;
     
-    AudioSystem::Get().SetBackend(std::make_unique<WASAPIBackend>());
+    AudioSystem::Get().SetBackend(std::make_unique<WASAPI::WASAPIBackend>());
     AudioSystem::Get().Initialize(config);
 }