Properly pass decoded excitation into LTP Synthesis

Before would discard excitation and always use a zero'ed out list

internal/silk/decoder.go

@@ -1359,6 +1359,13 @@ func (d *Decoder) ltpSynthesis(
 		return
 	}
 
+	// Voiced SILK frames, on the other hand, pass the excitation through an
+	// LTP filter using the parameters decoded in Section 4.2.7.6 to produce
+	// an LPC residual.
+	for i := range res {
+		res[i] = float32(eQ23[i]) / 8388608.0
+	}
+
 	// Voiced SILK frames, on the other hand, pass the excitation through an
 	// LTP filter using the parameters decoded in Section 4.2.7.6 to produce
 	// an LPC residual.