pcm/Volume: Open() returns output sample format

Prepare for a new mode which may convert to a different sample format
when applying volume, to reduce dithering.

test/test_pcm_volume.cxx

@@ -36,7 +36,7 @@ TestVolume(G g=G())
 	typedef typename Traits::value_type value_type;
 
 	PcmVolume pv;
-	pv.Open(F);
+	EXPECT_EQ(pv.Open(F), F);
 
 	constexpr size_t N = 509;
 	static value_type zero[N];