Thanks @M7HNF-Ian — this is a well-scoped, well-explained fix. The two changes mirror each other nicely and the comments in the code make the why obvious (Windows multi-host-API enumeration, MME latency rationale). A few notes:

Logic LGTM

• findPortAudioOutputDevice() — the refactor from "first match + ambiguity flag" to "collect all + pick WASAPI" is straightforward, and the behaviour for non-Windows or exactly-one-match cases is preserved. The paInDevelopment sentinel is a neat trick and the inline comment justifies it.
• defaultPortAudioOutputDevice() — the WASAPI preference pass on Windows is a clean mirror of the existing Linux JACK block. Nice symmetry.

One small observation (non-blocking)

start() at CwSidetonePortAudioSink.cpp:161–167 re-inspects the device's host API after defaultPortAudioOutputDevice() returns and sets m_fallbackOccurred = true / m_fallbackReason = "backend selected JACK default output" so the summary logger surfaces JACK selection as a backend substitution. The new WASAPI default-selection path doesn't get the same surfacing.

I think that's actually defensible — on Windows, WASAPI is the expected low-latency target, not an override the operator needs flagged the way JACK is on Linux — and you do emit a qCInfo at the selection site. Mentioning it only so you can confirm it's intentional. If you wanted parity, you'd add a qstrncmp(api->name, "Windows WASAPI", 14) == 0 arm to the same if (device.isNull()) block, but I don't think it's necessary.

Minor / cosmetic

• The Candidate::apiType field is populated on every platform but only read inside #ifdef Q_OS_WIN. Trivially wasteful (one PaHostApiTypeId per match candidate, on a list that's almost always tiny). Not worth changing.
• partials.size() - 1 in the qCInfo log expression is fine — we only enter that block when partials.size() >= 2, so the value can't be negative.

Scope / conventions

• Single file changed, exactly as described.
• No QSettings / AppSettings touched; no RAII surface; no new resources to leak.
• No null-deref risks introduced — every Pa_GetHostApiInfo / Pa_GetDeviceInfo return is checked before deref.
• The #ifdef Q_OS_WIN guards around the WASAPI-specific code keep this from affecting Linux/macOS builds.

Looks ready to me. 73 from a fellow CW operator-in-spirit — fast-keying jitter is a real annoyance and this should make a noticeable difference. Thanks for splitting it out of #3194 as requested.

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