In your sentence, "software" or "software tool" would sound better than "computer tool".
I assume that this context is the introduction to an academic paper. In this context, there are several possible meanings of "tool", including software, a paper-and-pencil evaluation tool, or even a mirror. As Will points out, "computer tool" is not idiomatic. Also, the phrase "aimed to" is not idiomatic. The word "software" means "computer program(s)", and can include anything from small pieces of a program to collections of many programs.
In your professional jargon, is "sound-to-letter skill" a singular noun that includes the skill of converting all sounds to letters? Or did you mean to use the plural "sound-to-letter skills", which treats transcriptions of different sounds as distinct skills?
I would change the sentence to something like one of these sentences:
In this paper, we present software that is designed to improve the sound-to-letter skills of ESL students.
In this paper, we present software that is intended to improve the sound-to-letter skills of ESL students.
In this paper, we present software that is aimed at improving the sound-to-letter skills of ESL students.