Sound fix diagram here. (pdf)

from Cristian Secară Homepage:

The first release (the +3) has a fatal hardware mistake: the 128K sound output is completely distorted.

• Tech explanation: the analog output of the AY-3-8912 sound chip was intended to be amplified by a one stage common-emitter transistor. In practice, they have forgot to put a resistor to the transistor’s emitter. The result: the transistor works in switch-mode – the sound is converted from analog multi-level amplitude to digitized two-level amplitude, same as the old 48K sound system.

source: secarica.ro ay8912 Demos

Autopsy:

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The original Plus2 (in the grey case) was released in 1987 to get away from the old Spectrums now that Amstrad had taken over. It was very different from previous Spectrums and had a proper typewriter keyboard, built-in casette recorder and 2 joystick ports.

For some unknown reason Amstrad later released a +2A in a black case which is actually a Plus3 with a casette recorder instead of a disk drive.

from Wikipedia:

The ZX Spectrum +2 was Amstrad’s first Spectrum, coming shortly after their purchase of the Spectrum range and “Sinclair” brand in 1986. The machine featured an all-new grey case featuring a spring-loaded keyboard, dual joystick ports, and a built-in cassette recorder dubbed the “Datacorder” (like the Amstrad CPC 464), but was in most respects identical to the ZX Spectrum 128. The main menu screen lacked the Spectrum 128′s “Tape Test” option, and the ROM was altered to account for a new 1986 Amstrad copyright message.

These changes resulted in minor incompatibility problems with software that accessed ROM routines at certain addresses. Production costs had been reduced and the retail price dropped to £139–£149.

The new keyboard did not include the BASIC keyword markings that were found on earlier Spectrums, except for the keywords LOAD, CODE and RUN which were useful for loading software. This was not a major issue however, as the +2 boasted a menu system, almost identical to the ZX Spectrum 128, where one could switch between 48k BASIC programming with the keywords, and 128k BASIC programming in which all words (keywords and otherwise) must be typed out in full (although the keywords are still stored internally as one character each).

Despite these changes, the layout remained identical to that of the 128.

source: wikipedia