In the recent history of English, there is a tendency to simplify the three-vowel alternation pattern in strong verbs with post-vocalic root nasals like begin, drink, ring, run, shrink, sing, sink, spring, stink, swim, such that the u-vowel of the past passive participle replaces the a-vowel of the preterit, especially in non-standard or colloquial speech. Here, for instance, is a contemporary example from the online edition of The New York Times:
“And although Mr. Rangel was not present, his booming voice still rung [instead of rang; emphasis added] through the hearing room as the committee lawyers played videos of him admitting to several of the charges during an impromptu speech he made on the House floor this summer, pleading for mercy from his colleagues.” (David Kocieniewski, “House Panel Says Facts in Rangel Case Are Undisputed,” November 15, 2010).
This process has already become normative in cling, fling, sling, slink, spin, sting, string, swing, and wring.
The directionality of the change should be noted: it is invariably the u-vowel of the past passive participle that replaces the a-vowel of the preterit, and never the other way around. Why? Because the /æ/ of rang, shrank, etc. is a marked vowel (marked for the feature of compactness), whereas the /ʌ/of rung, shrunk (cf. the 1989 movie title “Honey, I Shrunk the Kids”), etc. is an unmarked vowel (unmarked for the feature of compactness). The overarching tendency in language, whether in phonology or any other component of grammar, is for the marked unit to be replaced by its unmarked counterpart in a paradigm. The leveling of the ablaut pattern in English strong verbs conforms to just this tendency.
[Postscriptum, June 14, 2011: The degree to which the process of ablaut leveling has advanced in American English can be gauged by an example appearing on today’s Op-Ed page of The New York Times and written (nota bene) by someone identified as a professor of American studies and English at the Graduate Center of the City University of New York: “Unfortunately, these themes were lost in many of the stage versions . . . that sprung from its immense popularity.” (David S. Reynolds, “Rescuing The Real Uncle Tom,” New England Edition, p. A21)