The "hear" is critical to this verse, because this word lies in logical parallel to "the Lord our God."
The Masoretic Text contains cantillation marks which serve not only to indicate the stress and accents on the Hebrew words, but to provide musical harmony to "sing" the Scripture, which aids in memorization. The way the words are pronounced (based on the cantillation marks) therefore provides key logic markers on how to understand the verse.
For example, the command to "hear" is in direct harmonic parallel to the words "the Lord our God." Please click here, and then click here. (Please note that the two Tipcha disjunctive accents occur in parallel, since the verse is divided in half by the Atnach accent.) Please note therefore that on each side of the verse, the Tipcha accent (please see the red arrows) falls on the word for "hear" and on the words "the Lord our God."
So when heard in proper cantillation, the word "Israel" would be subsumed logically to "hear" (as indicated by the blue colored box here).
But the words "the Lord our God" would occur on the same note as "hear," and therefore the logical inference is Hear the Lord our God (because of the parallel Tipcha accents).
Finally, the words "the Lord is one" would be subsumed logically to "the Lord our God" (as indicated by the blue colored box here).
Thus the key thought of the verse --according to the musical annotations and accentuation within the Masoretic Text-- has to do therefore with listening to the Lord. In Biblical Hebrew, the verb to listen often appears in the context of obedience (i.e., "listening to the voice" of someone).
In summary, the Masoretic Text provides guidance for public readers of the Scripture, so that when the Scripture is read aloud (as in the case of this verse), an accent note, which is sung, occurs on both Tipchas in equal stress and cantillation force, so that the listeners will place both the "hear" and "the Lord our God" in logical parallel.
The question is asked in the comments, What is the Lord saying in Deuteronomy 6:4?
Answer: Obey (the voice of) the Lord. This answer stems from direct discussion found in the Babylonian Talmud.
In the Tractate Berakhot ("Blessings" in Talmud) we find guidance regarding the Shema (a section of the Torah recited as part of prayer), the Amidah (Silent prayer), and Birkat Hamazon (Grace after Meals). All three of these areas come into focus on whether the deaf-mute can "speak" and "hear" the Shema, and the answer is YES.
In other words, the Shema has as much to do with "speaking and listening" as is OBEYING, of which the rabbis contend the deaf-mute is capable. The OBEYING is therefore the emphasis. That is, what value would be the oneness of the Lord, if in fact you do not obey his voice?
In Berakhot 2:3, I.1.A, several rabbis discussed the rules of the Shema in the context of a deaf-mute. For example, the deaf-mute was not allowed to provide the heave-offering, but if he did so, the offering was acceptable. Since prayers after meals in silence were acceptable, and since the Shema could be recited in silence (but while standing), the deaf-mute could "speak" the Shema silently to himself.
The Talmud in this context concludes as follows...
“Matters follow the intention of the heart [and not what your lips speak, so one need not actually say the Shema audibly].”
In other words, what you do is just as important as what you say. The rabbis had thus contended "following teaching on Tannaite authority" that if the deaf-mute can obey the voice of the Lord through his actions (without "hearing" and "speaking" the Shema de novo), then this person is de facto "hearing" the Shema and is de facto "speaking" the Shema through his actions. Please click here to read this relevant portion of the Babylonian Talmud in English, which is cited from Neuser (2011).
In summary, the actions (of the deaf-mute who recites the Shema in his heart in silence, for example) will "speak" louder than spoken words of someone who actually hears and recites the Shema audibly, but who does not obey the Lord.
Neusner, J. (2011). The Babylonian Talmud: A Translation and Commentary (Vol. 1). Peabody, MA: Hendrickson Publishers, 95-97.