Total overhaul!

src/media-player-controller.js

@@ -0,0 +1,34 @@
+import {Controller} from "./controller.js";
+
+export class MediaPlayerController extends Controller {
+
+  get _value() {
+    return (this.stateObj.is_volume_muted === "on")
+    ? 0
+    : Math.ceil(this.stateObj.attributes.volume_level*100.0);
+  }
+
+  set _value(value) {
+    value = value/100.0;
+    this._hass.callService("media_player", "volume_set", {
+      entity_id: this.stateObj.entity_id,
+      volume_level: value,
+    });
+  }
+
+  get isOff() {
+    return this.stateObj.state === "off";
+  }
+
+  get string() {
+    if (this.stateObj.attributes.is_volume_muted)
+      return "-";
+    return !!this.stateObj.attributes.volume_level
+      ? `${this.value} %`
+      : this._hass.localize("state.media_player.off");
+  }
+
+  get hasToggle() {
+    return false;
+  }
+}