Convert WMV to MP4
WMV is a Microsoft container built around WMV9 video and WMA audio - codecs MP4 doesn't accept. Wave re-encodes both streams locally using LGPL codecs and hands you an MP4 that plays anywhere.
What actually changes
WMV files use Microsoft's Advanced Systems Format container and almost always carry WMV9 video plus WMA audio. Neither codec is compatible with MP4, so the conversion is always a full re-encode - there is no fast-copy path from a WMV.
Because your browser can't decode WMV natively, Wave takes a two-step route: the on-device ffmpeg-wasm engine decodes the WMV frames, and the browser's WebCodecs H.264 encoder (hardware-accelerated on nearly every modern machine) writes the H.264 output. Audio re-encodes to AAC at 192 kbps. On the rare browser without WebCodecs H.264, Wave falls back to the LGPL MPEG-4 Part 2 encoder inside the wasm core. The result plays natively on iOS, macOS, Android, Windows, and every browser. If your source is protected by Windows DRM (Plays For Sure), Wave can't decode it - no browser-side converter can.
Three steps.
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Step 1
Drop the WMV file onto the card, or click Pick a video.
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Step 2
Wave inspects the file and shows Re-encode needed - WMV always takes this path.
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Step 3
Convert and save the MP4.
Answers, specifically.
Will the MP4 be smaller than the WMV?
Usually similar in size, sometimes slightly larger. WMV9 was a competitive codec in its era; the LGPL video encoder Wave uses is a shade less efficient at the same visual quality.
Why is WMV to MP4 slower than MKV to MP4?
Because WMV can't be repackaged into MP4 - the codecs are incompatible. Every frame has to be decoded and re-encoded.
My WMV won't play in QuickTime. Will the MP4?
Yes. That's the main reason to convert WMV: broad player support. MP4 with LGPL-encoded video plus AAC audio plays in QuickTime, iOS, macOS, and every browser.
Is DRM-protected WMV supported?
No. Windows-Media DRM decoding isn't possible outside Microsoft's own player, and Wave - running purely in the browser - has no way around that.
Does the audio quality drop noticeably?
The audio re-encodes from WMA to AAC at 160 kbps. That's transparent for most music and voice; audiophile source material may lose a small edge.