The Zohar writes, that whoever tells over what occurred by yetzias mitzrayim and celebrates it joyfully, will merit being joyous with the Shechinah in Olam Haba. The Arizal said that the haggada should be recited loudly, with great simcha and proper kavana.
The attendant Reb Nachman Maryashin related that when he was once at the seder of the Rebbe Maharash, and all were sitting with awe and derech eretz, saying the haggada quietly, the Rebbe instructed that the haggada be said in a loud voice.
The Frierdiker Rebbe also said it in a loud voice, and when he became physically unable to do so, he asked others to say it aloud. The Rebbe as well would motion to the one reading the haggada to read it out loud. During Hallel, the Rebbe would read aloud with a sweet tune and a noticeable dveikus, and tears would flow from his eyes.
(אוצר מנהגי חב”ד ע’ קנד, קצח)