4 sentences

1. 远处传来了沉闷的雷声。

   yuǎn​chù chuán​lái le chén​mèn de léi​shēng.

   Dull rolls of thunder could be heard in the distance.

   Source: Dong Chinese Dictionary

2. 他感觉自己的心口窝有一阵沉闷的疼痛。

   tā gǎn​jué zì​jǐ de xīn​kǒu wō yǒu yī zhèn chén​mèn de téng​tòng.

   He felt a dull pain in the pit of his stomach.

   Source: Dong Chinese Dictionary

3. 他这两天心情沉闷，一句话也不想说。

   tā zhè liǎng tiān xīn​qíng chén​mèn, yī jù huà yě bù xiǎng shuō.

   He has been in a gloomy mood for the last couple of days and doesn't want to say a word.

   Source: Dong Chinese Dictionary

4. 窗外天空阴暗，天气沉闷得让人感到有些喘不过气。

   chuāng wài tiān​kōng yīn'àn, tiān​qì chén​mèn de ràng rén gǎn​dào yǒu​xiē chuǎn bù guò qì.

   The sky outside was gloomy, and the weather was so stifling that it felt almost suffocating.

   Source: Dong Chinese Dictionary