Add alt title for audio button.

config/locales/rucaptcha.en.yml

@@ -3,3 +3,4 @@ en:
     invalid: "The captcha code is incorrect (if you can't read, you can click image to refresh it)"
     placeholder: please fill in the identifying code
     captcha: Captcha
+    play_captcha_voice: "Play Captcha Voice"

config/locales/rucaptcha.zh-CN.yml

@@ -1,3 +1,4 @@
 'zh-CN':
   rucaptcha:
     invalid: "验证码不正确（如无法识别，可以点击刷新验证码）"
+    play_captcha_voice: "播放验证码的语音"

config/locales/zh_tw.yml

@@ -1,4 +1,5 @@
 zh_tw:
   rucaptcha:
     placeholder: 請填寫驗證碼
-    captcha: 驗證碼
+    captcha: 驗證碼
+    play_captcha_voice: 播放驗證碼語音

lib/rucaptcha/view_helpers.rb

@@ -22,7 +22,7 @@ module RuCaptcha
       tag(:img, opts)
     end
     def rucaptcha_audio_tag()
-      "<button type=\"button\" onclick=\"document.getElementById(&quot;captcha_audio&quot;).play();\" class=\"fas fa-volume-up\" style=\"font-size: 1.5em; color: #333; cursor: pointer; border: 2px solid #333; padding: 0.2em;\"></button><audio id=\"captcha_audio\" src=\"#{ru_captcha.root_path(:format=>:wav)}\"></audio>".html_safe
+      "<button title=\"#{I18n.t('rucaptcha.play_captcha_voice')}\" type=\"button\" onclick=\"document.getElementById(&quot;captcha_audio&quot;).play();\" class=\"fas fa-volume-up\" style=\"font-size: 1.5em; color: #333; cursor: pointer; border: 2px solid #333; padding: 0.2em;\"></button><audio id=\"captcha_audio\" src=\"#{ru_captcha.root_path(:format=>:wav)}\"></audio>".html_safe
     end
     def gotcha(opts = {})
       if opts[:placeholder].blank?