“变低沉”可以用以下几种英语表达，具体取决于语境：
1、 Become low-pitched：这个表达侧重于描述声音的音高变低，适用于音乐、语音等场景中音调降低的情况。例如，“His voice became low-pitched as he spoke more seriously.”（他说话时语气变得严肃，声音也变低了。）
2、 Turn somber/gloomy：这两个表达更侧重于描述情绪或氛围的低沉、阴郁。例如，“The atmosphere in the room turned somber after the news was announced.”（消息宣布后，房间里的气氛变得低沉了。）或者“Her mood turned gloomy as the day wore on.”（随着一天过去，她的心情变得低沉了。）
3、 Sink into a low tone：这个表达形象地描述了声音或情绪逐渐降低到低沉的状态。例如，“He sank into a low tone as he shared his worries.”（他分享自己的担忧时，声音逐渐变低了。）
4、 Grow muted/subdued：这两个表达可以描述声音或情绪变得不那么响亮、活跃，带有一种低沉、压抑的感觉。例如，“The crowd grew muted as the speaker continued.”（随着演讲者的继续，人群变得安静了，气氛也有些低沉。）或者“Her laughter was subdued by the serious news.”（这个严肃的消息让她的笑声变得低沉了。）