Does sour milk have something else in it besides lactic acid that gives it such a horrific taste and odor? Even slightly sour milk has a horrible, nauseating taste, while plain yogurt tastes either great, or at least nowhere near as bad. A dozen sources (all of them) say that the sour odor and taste of sour milk and also of yogurt is caused by the presence of lactic acid. Does sour milk have something else in it besides lactic acid that gives it the horrific taste and odor that yogurt (which has plenty of lactic acid it it) almost never has. Or is there more than one kind of lactic acid?
Is the taste of sour milk due to lactic acid and one or more amino acids?
https://foodsafety.foodscience.cornell.edu/sites/foodsafety.foodscience.cornell.edu/files/shared/documents/CU-DFScience-Notes-Milk-Flavor-Defects-Sensory-Eval-04-10.pdf has this to say:
&quot;CHARACTERISTICS OF MILK OFF-FLAVORS...
Typical Milk:
No criticism. Very little distinct odor, pleasantly sweet and clean with no aftertaste...
Acid:
Basic taste sensation. Sour, tart, may cause tingling sensation on tongue. “Cultured milk” or “sour” odor may be present. Cause: Growth  of  lactic  acid  producing  organisms  such  as Lactococcus lactis,  due  to  poor refrigeration, especially when temperatures exceed 70°F (21°C).&quot;
Like everyone, the Cornell author explains the sour taste and odor of sour milk by referencing only lactic acid.
https://www.nationalgeographic.com/culture/article/when-good-milk-goes-bad is no different when talking about slightly sour milk, as opposed to the much later (very) spoiled moldy milk, which involves yeasts and molds and ...danger.
https://www.reddit.com/r/chemistry/comments/j15maj/what_does_lactic_acid_smell_like/ says pure lactic acid does not smell bad. Aha! On the other hand, it does say whether or not it smells like sour milk. What a pity.
To those that nitpick that it is subjective to say that sour milk tastes awful, I say this: 1. Taste and odor is subjective, obviously. 2. I Googled 'Sour milk tastes okay' and there was not one relevant hit so people are not in any way divided on this. 3. This is an SE question, not an academic paper.
Probably. Lactic acid bacteria produce other volatile compounds such as acetaldehyde and diacetyl that could be perceived as off-flavors. Both where fermentation is the objective and in the case of spoilage, the VOC profile can vary with the identity of the responsible bacteria but also of the milk and its treatment.
For instance, in Ref. 1 they discuss the effect of different lactic acid bacteria:

During fermentation, S. thermophilus and L. delbrueckii ssp. bulgaricus generate lactic acid and a variety of volatile organic aroma compounds. These compounds impart particular flavors to fermented milk (Beshkova et al., 1998). The overall flavor of any fermented milk product is formed by a large number of these volatile compounds. Among these compounds, carbonyl compounds and organic acids such as acetaldehyde, diacetyl, and acetic acid play a crucial role and can be used to evaluate the flavor quality of dairy products (Zha et al., 2015). Ott et al. (1997) found that acetaldehyde provided the typical flavor of dairy products such as yogurt and buttermilk. Erkus et al. (2013) also reported that diacetyl contributed the characteristic sour buttery flavor to some fermented dairy products.

The offending chemical could also be vinegar (acetic acid). Again from Ref. 1:

Acetic acid has been found previously as a volatile in yogurt by Panagiotidis et al. (2001). High levels of acetic acid contributes most to the tart flavor of yogurt and is particularly associated with a “vinegary, pungent, acidic” taste (Panagiotidis et al., 2001).

If you search for &quot;chemical profile of spoiled milk gc&quot; you'll find a wealth of references discussing the effect of different milk (due to species, handling or forage), milk treatment, and spoilage bacteria on the VOC profile.
In addition, what makes the milk taste bad may depend on the bacteria and on you. That makes narrowing the answer down to a particular compound impossible.
References

T. Dan, D. Wang, R.L. Jin, H.P. Zhang, T.T. Zhou, T.S. Sun, Characterization of volatile compounds in fermented milk using solid-phase microextraction methods coupled with gas chromatography-mass spectrometry, Journal of Dairy Science, Volume 100, Issue 4, 2017, Pages 2488-2500, ISSN 0022-0302, https://doi.org/10.3168/jds.2016-11528.