Add TTS volume, a browser preview, and the split-audio plan

poc/src/audio.ts

@@ -276,7 +276,7 @@ export class AudioManager {
         this.modules = [];
     }
 
-    async play(file: string): Promise<void> {
+    async play(file: string, volume?: number): Promise<void> {
         if (!this.available) throw new AudioUnavailableError(this.error ?? "audio is not available");
 
         const binary = process.env.VESKSLOP_FFPLAY ?? "ffplay";
@@ -284,7 +284,17 @@ export class AudioManager {
         // Lead-in silence: give Discord's voice pipeline a moment to settle
         // before the first syllable, so the attack is not swallowed.
         const args = ["-nodisp", "-autoexit", "-loglevel", "error"];
-        if (LEAD_MS > 0) args.push("-af", `adelay=${LEAD_MS}:all=1`);
+        const filters: string[] = [];
+        if (LEAD_MS > 0) filters.push(`adelay=${LEAD_MS}:all=1`);
+
+        // Volume percent, 0-200. 100 keeps the level as synthesized. ffplay's own
+        // -volume stops at 100, so use the volume filter and its dB scale.
+        if (typeof volume === "number" && Number.isFinite(volume) && volume !== 100) {
+            const gain = volume > 0 ? `${(20 * Math.log10(volume / 100)).toFixed(2)}dB` : "0";
+            filters.push(`volume=${gain}`);
+        }
+
+        if (filters.length) args.push("-af", filters.join(","));
         args.push(file);
 
         await new Promise<void>((resolve, reject) => {