SPU: Run muted voices when IRQ is enabled

Fixes SPU timeout in Casper. We can still optimize for most games where
interrupts are not used by completely ignoring muted voices in those.

src/core/spu.cpp

@@ -1225,7 +1225,7 @@ void SPU::ReadADPCMBlock(u16 address, ADPCMBlock* block)
 std::tuple<s32, s32> SPU::SampleVoice(u32 voice_index)
 {
   Voice& voice = m_voices[voice_index];
-  if (!voice.IsOn())
+  if (!voice.IsOn() && !m_SPUCNT.irq9_enable)
   {
     voice.last_volume = 0;
     return {};
@@ -1245,18 +1245,28 @@ std::tuple<s32, s32> SPU::SampleVoice(u32 voice_index)
     }
   }
 
-  // interpolate/sample and apply ADSR volume
-  s16 sample;
-  if (IsVoiceNoiseEnabled(voice_index))
-    sample = GetVoiceNoiseLevel();
+  // skip interpolation when the volume is muted anyway
+  s32 volume;
+  if (voice.regs.adsr_volume != 0)
+  {
+    // interpolate/sample and apply ADSR volume
+    s16 sample;
+    if (IsVoiceNoiseEnabled(voice_index))
+      sample = GetVoiceNoiseLevel();
+    else
+      sample = voice.Interpolate();
+
+    volume = ApplyVolume(sample, voice.regs.adsr_volume);
+  }
   else
-    sample = voice.Interpolate();
+  {
+    volume = 0;
+  }
 
-  const s32 volume = ApplyVolume(sample, voice.regs.adsr_volume);
-  // if (voice_index == 3 || voice_index == 4)
-  // Log_WarningPrintf("voice %u %d", voice_index, volume);
   voice.last_volume = volume;
-  voice.TickADSR();
+
+  if (voice.adsr_phase != ADSRPhase::Off)
+    voice.TickADSR();
 
   // Pitch modulation
   u16 step = voice.regs.adpcm_sample_rate;