All dictionary entries put 什么 as shen2me0, but I am quite sure that in fluid speech I've only ever heard shen3me0. What's going on here? Is this a dialect/regional accent thing? For context, I grew up in Beijing.
The shen3 is in fact not a shen3, but an unstressed shen2. Its tone numeral is 35, but weakened to a 3.
The me0 is a weakened me1. Its tone numeral is 5 (the so-called neutral tone, essentially an unstressed 55).
So the "shen3me0" is in fact 3->5.
The unstressed 3 sounds close to the third tone with tone numeral 214. The overall contour of the third tone is a dipping one but 214 is sometimes weakened to a flat 3.
Meanwhile, shen2me0 is 35->31.
So why not just pronounce it as 35->31?
- A possible reason: shenme is often used to raise a question.
When raising a question we unconsciously raise the tone of our voice. This in turn makes the pitch of syllables at the beginning of a sentence/phrase lower than the pitch of those at the end. When the first syllable is a 35 (the second tone) and second syllable a 5, the first syllable is lowered and weakened to a 3, making it sound like the unstressed third tone.
- Another possible reason: shenme appears in weishenme.
The standard pronunciation for weishenme is wei4shen2me, but it is often pronounced as wei4shen0me0. The shen is weakened to shen0. Its tone numeral is 3 and again, close to the weakened version of the unstressed third tone. Some would consider the pronunciation of every shenme to be the same as the pronunciation of shenme in weishenme.
- A third possible reason: the speaker's native tone.
In some languages/dialects such as Lower Yangtze Mandarin, Gan, Xiang, Southern Min, Cantonese, and a few Wu varieties, either 什 or 甚 has a relatively low and flat pitch and sounds similar to the unstressed third tone in Mandarin.
Note that these 2 different characters with different pronunciations 什 甚 are used for the same meaning. That's a whole other story.