Quoth the Wikipedia...

In June of 1983, Sony introduced high fidelity audio to videotape as Beta Hi-Fi. For NTSC, Beta HiFi worked by placing a pair of FM carriers between the chroma (C) and luminance (Y) carriers, a process known as frequency multiplexing. Each head had a specific pair of carriers; in total, four individual channels were employed. Head A recorded its hi-fi carriers at 1.38(L) and 1.68(R) MHz, and the B head employed 1.53 and 1.83 MHz. The result was audio with an 80 dB dynamic range, with less than 0.005% wow and flutter.

Prior to the introduction of Beta Hi-Fi, Sony shifted the Y carrier up by 400 kHz to make room for the 4 FM carriers that would be needed for Beta Hi-Fi. All Beta machines incorporated this change, plus the ability to hunt for a lower frequency pre-AFM Y carrier. Sony incorporated an "antihunt" circuit, to stop the machine hunting for a Y carrier that wasn't there.

Some Sony NTSC models were marketed as "Hi-Fi Ready" (with an SL-HFR prefix to the model's number instead of the usual SL or SL-HF). These Betamax decks looked like a regular Betamax model, except for a special 28-pin connector on the rear. If the user desired a Beta Hi-Fi model but lacked the funds at the time, he could purchase an "SL-HFRxx" and at a later date purchase the separate Hi-Fi Processor. Sony offered two outboard Beta Hi-Fi processors, the HFP-100 and HFP-200. They were identical except that the HFP-200 was capable of multi-channel TV sound, with the word "stereocast" printed after the Beta Hi-Fi logo. This was possible because unlike a VHS Hi-Fi deck, an NTSC Betamax didn't need an extra pair of heads. The HFP-x00 would generate the needed carriers which would be recorded by the attached deck, and during playback the AFM carriers would be passed to the HFP-x00. They also had a small "fine tracking" control on the rear panel for difficult tapes.

For PAL, however, the bandwidth between the chroma and luminance carriers was not sufficient to allow additional FM carriers, so depth multiplexing was employed, wherein the audio track would be recorded in the same way that the video track was. The lower-frequency audio track was written first by a dedicated head, and the video track recorded on top by the video head. The head disk had an extra pair of audio-only heads with a different azimuth, positioned slightly ahead of the regular video heads, for this purpose.

Sony was confident that VHS could not achieve the same audio performance feat as Beta Hi-Fi. However, to the chagrin of Sony, JVC did develop a VHS hi-fi system on the principle of depth multiplexing approximately a year after the first Beta Hi-Fi VCR, the SL-5200, was introduced by Sony. Despite initial praise as providing "CD sound quality", both Beta Hi-Fi and VHS HiFi suffered from "carrier buzz", where high frequency information bled into the audio carriers, creating momentary "buzzing" and other audio flaws. Both systems also used companding noise-reduction systems, which could create "pumping" artifacts under some conditions. Both formats also suffered from interchange problems, where tapes made on one machine did not always play back well on other machines. When this happened and if the artifacts became too distracting, users were forced to revert to the old linear soundtrack.