diff --git a/src/synthesizer/index.ts b/src/synthesizer/index.ts
index 66f1320..89adde0 100644
--- a/src/synthesizer/index.ts
+++ b/src/synthesizer/index.ts
@@ -34,7 +34,7 @@ export default class Synthesizer {
       body: `
 <speak version=\"1.0\" xmlns=\"http://www.w3.org/2001/10/synthesis\" xml:lang=\"ja-JP\">\
 <voice name=\"${this.voice}\">\
-  <prosody rate=\"${this.speed}\">\
+  <prosody rate=\"${this.speed}\" volume=\"75\">\
     ${text.replaceAll(/["&'<>"]/g, (match: string) => {
       return escapeMap[match] ?? "";
     })}\