"At" vs. "in" before verb

Solution 1:

Here are the first 25 COCA results for Adjective + in understanding (the number on the right is the number of hits for a sequence). In all of these the adjective preceding the prepositional phrase characterizes instrument which leads to understanding rather than characterizing some person's ability at understanding.

1   HELPFUL IN UNDERSTANDING THE      26
2   USEFUL IN UNDERSTANDING THE       24
3   INTERESTED IN UNDERSTANDING THE   23
4   IMPORTANT IN UNDERSTANDING THE    15
5   HELPFUL IN UNDERSTANDING HOW       8
6   INTERESTED IN UNDERSTANDING HOW    5
7   IMPORTANT IN UNDERSTANDING AND     4
8   CRITICAL IN UNDERSTANDING THE      4
9   INVOLVED IN UNDERSTANDING THE      4    
10  USEFUL IN UNDERSTANDING HOW        4
11  IMPORTANT IN UNDERSTANDING HOW     3
12  HELPFUL IN UNDERSTANDING GLOBAL    3
13  CRUCIAL IN UNDERSTANDING THE       3
14  INTERESTED IN UNDERSTANDING THEIR  3
15  INTERESTED IN UNDERSTANDING WHAT   3
16  INVOLVED IN UNDERSTANDING OR       3
17  USEFUL IN UNDERSTANDING WHY        3
18  BENEFICIAL IN UNDERSTANDING THE    2
19  ESSENTIAL IN UNDERSTANDING THE     2
20  CRITICAL IN UNDERSTANDING HOW      2
21  INTERESTED IN UNDERSTANDING AND    2
22  HELPFUL IN UNDERSTANDING WHY       2
23  HELPFUL IN UNDERSTANDING WHAT      2
24  NECESSARY IN UNDERSTANDING WHAT    2
25  PIVOTAL IN UNDERSTANDING THE       2

Now see the results for Adjective + at understanding. In all of these, the adjective characterizes a person's ability to understand.

1   GOOD AT UNDERSTANDING THE              4    
2   SUCCESSFUL AT UNDERSTANDING MICROBIAL  1    
3   SLOW AT UNDERSTANDING AND              1
4   POOR AT UNDERSTANDING ECOLOGICAL       1
5   INCOMPETENT AT UNDERSTANDING HOW       1    
6   GOOD AT UNDERSTANDING WHY              1
7   GOOD AT UNDERSTANDING THEIR            1
8   GOOD AT UNDERSTANDING TECHNICAL        1
9   GOOD AT UNDERSTANDING RIDDLES          1
10  GOOD AT UNDERSTANDING OR               1
11  GOOD AT UNDERSTANDING HOW              1
12  GOOD AT UNDERSTANDING GLOBAL           1
13  BETTER AT UNDERSTANDING DIFFERENT      1
14  BETTER AT UNDERSTANDING A              1
15  BAD AT UNDERSTANDING WHAT              1
16  BAD AT UNDERSTANDING HOW               1
17  ADEPT AT UNDERSTANDING THEIR           1
18  ADEPT AT UNDERSTANDING AND             1

It wouldn't be safe, however, to assume that this pattern holds for any given adjective + PP[pres.ppl] combination. When we search for skilled + (in/at) + V.pres.ppl we find 109 hits for skilled at V-ing and 173 hits for skilled in V-ing. Not much of a discrepancy as compared to the clear differentiation between in understanding and at understanding.

Solution 2:

Although I don't think it's standard American English to use "in", I do think it would be understandable to any native speaker of English, but I'd use "at" here instead of "in" just because it sounds "right" to my ear. "He's good at (playing) the piano" is normal, but "He's good in (playing) the piano" is not. "He's good at math" is normal, as is "He's good in math". The structure "to be good at [doing something]" is a standard idiom in English. "She's good at horseback riding" but "She's good in bed" are normal. "He's good at French" means that he's a good French speaker, and maybe "He's good in French" means that when he says things like that in French, they're funnier or more poetic or more interesting than when he says them in English.

Solution 3:

We generally use at with reference to a verb/ gerund, in with a noun. This is not a rule of grammar -- not even a necessary requirement, though.

He is good at playing football.
He is good in sports.

Also, from the same example above, we can see that in agrees with a broader subject (sports) compared with at, which is used with a well-defined, focused subject (football).

"... more accurate at understanding speech ..." is therefore, correct and better than with in.