- Is your machine locale changed to Japanese with Decimal being set to a period [.]? Changing decimal is only necessary if you have a non-USA version of Windows (Primarily an issue with European computers).
- Do you have the Lyric Diphonizer (convert to VCV only), Iroiro (converts between romanized, kana, CV, or VCV), or AutoCVVC (converts to CV, CVVC, and VCV based on user created dictionaries) plugins?
- Are the USTs you're trying formatted properly for the respective bank formats (CV, CVVC, VCV, romanized, kana, etc.) and are the voicebanks fitted (Ctrl+E, erase preutterance, overlap, consonant velocity, BRE, flags, and STP)?
- Are you seeing gibberish symbols, question marks, etc. (if so, then that answers the first question about locale being set or not)?
The default voice (Defoko), Yokune Ruko, and Namine Ritsu are Japanese banks and are only aliased to read USTs in hiragana. Defoko only reads hiragana CV USTs. IDK which Ruko bank you downloaded, but chances are high that it's a VCV bank. USTs require conversion to the VCV format with this type of bank if you're not using UTAU's shareware version. Namine Ritsu CVVC reads CVVC usts in hiragana and like Ruko, requires conversion before it will properly work.
All 3 of the mentioned banks will not work and will have corrupted filenames if your machine has not been set up properly before installing UTAU. UTAU - by extension - will have messed up coding and will require a new download and installation if you installed it before your machine was set to Japanese locale and rebooted.
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As for making your own voicebank from scratch; UTAU has the least trouble with .wav files set to 16-bit, 44100kHz and ideally with as little noise (digital or ambient) as possible.