A word that means someone is losing attention to the conversation due to being too far from the speaker

Perhaps the term "physical remoteness" will approximate the idea sufficiently well.

(SOED) 3a Not closely connected with or affecting a person or thing. Late 16th century.
♦ M.L. KING Many whites to whom the horror of slavery had been emotionally remote.
♦ M. LEITCH A place as remote from her own concerns as Siberia.

As the examples show this term implies distance either time-wise, space-wise or figuratively and so it is preferable apparently to specify the level considered by adding the modifier "physical".

• He couldn't understand the presentation because he was affected by the physical remoteness of the speaker.

(Technology, Morality, and Social Policy) But remoteness is more than just spatial remoteness, and if we generalize these insights, we can see other relevant […]

(Human Communication Research - Volume 10, Issue 2 - Page 210) By using conflicts varying in their degree of remoteness, it was possible to examine not only the overall differences in the perception of the two worlds but simultaneously how the degree of remoteness affects these perceptions.

(Estrangement: Marx's Conception of Human Nature and the) As used in popular language today, the term designates individual or group remoteness from and/or disaffection with something or someone. It may therefore be said that “ alienation ” stands for a discrepancy of some sort.

Out of earshot, not within earshot

When you can't hear something because of your distance to the speaker, you are not within earshot. It's easier to focus on conversations that are within earshot.

Cambridge gives

earshot, n: From the range of distance within which it is possible to be heard or to hear what someone is saying

Your example could be completed as:

He couldn't understand the presentation because it was out of earshot.

If only his seat had been closer, he would have been engaged like the others within earshot of the speaker.

Those within (or in) earshot can hear and be heard well.

How about "zoning out"? It's a little general, yes, but that is what the listener is doing.