Hi, Sarah, That is really a professonal question. As a native Chinese speaker, I can't really tell the differnce of "e" sound between 可 and 什么 What you need to pay attention 么 is used pronunced as 轻声/untoned, that is probably why you think it the other way as it is short and unstressed as in the schwa of "sofa" in English. I hope it helps! So to answer your observation, I think you are right by saying so.

In mandarin, they are of the same phoneme, in other words, they sound "identical" to "us"/chinese speakers.

There is no exact equivalent of the schwa sound [ə] in Mandarin Chinese. Mandarin Chinese has a relatively small set of vowels, and the mid central vowel [ə] is not among them. The closest Mandarin Chinese vowel to schwa would be [ɤ], which is a mid back unrounded vowel. It is written as 'e' in the Pinyin romanization system. This vowel [ɤ] can appear in unstressed syllables in Mandarin, similar to the role of schwa in English. However, there are some key differences: [ɤ] can still carry one of the four Mandarin tones, whereas schwa is typically toneless. [ɤ] does not reduce or centralize other vowels in Mandarin the way schwa does in English. The Mandarin vowel system remains largely intact in unstressed syllables. [ɤ] does not have the same degree of variability and contextual dependence as schwa. It has a more consistent quality across contexts. So in summary, while [ɤ] shares some of the phonetic and phonological behavior of schwa, there are also some significant differences. Mandarin lacks a true equivalent of the schwa found in English and some other languages.

汉语的e和英语的schwa是不同的两个音，这是肯定的，不然国际音标就不会用两个了。我英语不好，没法用英语解释。

么 is like "ma"" in machine；的，"de" in code