Rashi: Refer to 5:2:2:2.

Ibn Ezra: When many matters are mixed in a dream, it is from a mixture of foods, and one of the four 'elements' (earth, fire, wind and water) overpowered. It will not have an interpretation, and it is not a good or bad omen; it is Hevel.

Rid: This refers to "Al Tevahel Al Picha" (1). Be careful not to be a Kesil in your Neder. Just like a dream comes with many matters, a Kesil's voice comes with many idle matters. Therefore, do not be like a Kesil in your vow.

Seforno: A dream comes with many idle matters; usually it resembles what he thought about during the day - he thought the entire day, and at night saw it [in a dream - Brachos 56a] .

Rav Sadya Gaon: Just like dreams have many matters, a fool's voice has a multitude of words.

Rashi: A dream generally comes via many thoughts that a person thinks about during the day ¹ (Brachos 55b). So a voice of folly comes from the many words that a Kesil says; "[b'Rov Devarim] Lo Yechdal Pesha" (Mishlei 10:19).

Ibn Ezra: Just like a dream with many matters is mere Hevel, so is a Kesil who speaks much. Increase of words always leads to sin (Avos 1:17), and all the more so in a place prepared to mention Hashem's name constantly.

Rashbam, Ri Kara: Just like a dream that comes amidst many thoughts during the day is folly, so from a Kesil's many words it is known and recognized that he is a Kesil. One should minimize words, to distance from Kesilus!

Refer to 5:2:1:3.

Seforno: Just like a dream has idle matters resembling his thoughts, so a Kesil who prays at length, imagination will overpower and he will veer from intent.

Metzudas David: Just like a dream normally comes to one who thinks about many things, so the voice of Kesilus of complaints comes in one who increases words.