Displaying IPA symbols in Beamer

You’ve run into a gotcha with beamer and fontspec The setmainfont command in fontspec sets the serif family, but beamer changes the default font to the sans-serif family. So, your document is not using the font you selected with setmainfont.

If you check the warning messages tracinglostchars2 gives you on the console, you’ll see it telling you that the current font is Latin Modern Sans 10 and it does not have those characters. That’s the default sans-serif font fontspec loads, and the MWE never changed it.

You have a few options. One is to load a font family that has these symbols, such as DejaVu or Libertinus.

documentclass{beamer}

tracinglostchars=2
usepackage{fontspec}
setmainfont{Arial}

usepackage{fontspec}
setmainfont{DejaVu Serif}
setsansfont{DejaVu Sans}
setmonofont{DejaVu Sans Mono}

begin{document}

title{Title}   
author{} 
date{} 

frame{frametitle{lí-hó} 

ɕʰ sʰ ʑ  ɾ  k̚  t̚  p̚  ʔ pʰ tʰ kʰ

lí kám ē-hiáu khòaⁿ chiah ê jī
}

end{document}

This will use DejaVu Sans by default, but let you use rmfamily, ttfamily or textrm to switch to serif or monospaced fonts that also have those symbols. Remember to check for warnings about missing characters!

Another solution, which Alan Xiang brought up in the comments, is to add the line usefonttheme{serif} to your preamble. This will make setmainfont work the way you expect.

Finally, you can set the missing Unicode characters active and supply them from another font. This example uses the free version of URW Charter as the main font, but takes all the linguistic symbols it’s missing from Charis SIL (which is based on Charter).

documentclass{beamer}

tracinglostchars=2
usepackage{fontspec}
setmainfont{Arial}

usepackage{fontspec}
usepackage{newunicodechar}

usefonttheme{serif}

setmainfont{XCharter}
newfontfamilyipafont{Charis SIL}[
  Ligatures=Common,
  Scale=MatchLowercase ]

newunicodechar{^^^^0255}{{ipafontsymbol{"0255}}}
newunicodechar{^^^^027e}{{ipafontsymbol{"027E}}}
newunicodechar{^^^^0291}{{ipafontsymbol{"0291}}}
newunicodechar{^^^^0294}{{ipafontsymbol{"0294}}}
newunicodechar{^^^^02b0}{{ipafontsymbol{"02B0}}}
newunicodechar{^^^^031a}{{ipafontsymbol{"031A}}}
newunicodechar{^^^^207f}{{ipafontsymbol{"207F}}}

begin{document}

title{Title}   
author{} 
date{} 

frame{frametitle{lí-hó} 

ɕʰ sʰ ʑ  ɾ  k̚  t̚  p̚  ʔ pʰ tʰ kʰ

lí kám ē-hiáu khòaⁿ chiah ê jī
}

end{document}