examples, lang: funcs: core: example: Update vumeter

This patch moves to use the sox package instead of arecord for getting
microphone data, and it also validates that both sox and rec and
installed. We also add a standalone example.

examples/lang/vumeter.mcl

@@ -0,0 +1,8 @@
+import "example"
+
+$vumeter = example.vumeter("====", 10, 0.9)
+
+file "/tmp/vumeter" {
+	state => $const.res.file.state.exists,
+	content => $vumeter,
+}