How did long-term contact with Mon-Khmer languages affect Chamic languages?

Long-term contact with Mon-Khmer languages significantly influenced the Chamic languages through generations of bilingualism, leading to notable structural adaptations.1

The most prominent effect was the acquisition of two distinct voice registers—a "breathy" or "sepulchral" voice and a "clear" voice—which are characteristic of many Austroasiatic languages within the Mon-Khmer family. These areal adaptations were so significant that, in 1906, the linguist Wilhelm Schmidt incorrectly classified the Chamic group as "Austroasiatic mixed languages" rather than recognizing their true status as Austronesian languages.12

In addition to these phonological changes, some Chamic languages have undergone further shifts due to contact with other linguistic groups. For instance, where Chamic languages have been exposed to languages with lexical tone, such as Vietnamese, they have become largely monosyllabic and tonal.1