What is the significance of the statement "va'Yishma Elokim El Leah", particularly bearing in mind that the Torah does not say that she prayed?
Seforno: We learn from here that when one makes the necessary Hishtadlus (like Leah did) and combines it with Tefilah - which is (perhaps) taken for granted that that is what Leah did, 1 Hashem responds favorably. 2
Ohr ha'Chayim: she did not pray. Hashem 'heard' (knew) that she intended for the Mitzvah, and went overboard amidst love of Hashem. Due to this, Hashem went overboard and gave to her a fifth son (she was supposed to have only four) and a sixth, Midah k'Neged Midah. 3
Ha'Emek Davar: This teaches that also Leah did not act for pleasure, rather, as prayer and preparation for children.
See also Sifsei Chachamim.
Or ha'Ganuz, in Likutei Ha'aros on Ohr ha'Chayim: Leah went overboard in (a) giving her Shifchah to Yakov, and (b) going out to greet him. Amidst modesty, she did not allude to this when she named her second extra son. (Even if initially she thought that Yisachar was due to both of these (refer to 30:18:1:1), when she later gave birth to Zevulun, she realized that each son was for one of these.
What was the gist of Leah's Tefilah?