  We present \emph{Spitzer} IRS data on 19 asymptotic giant branch (AGB) stars
in the Large Magellanic Cloud, complementing existing published data sets of
carbon stars in both Magellanic Clouds and the Milky Way, to investigate the
effects of metallicity on dust and molecular spectral features arising from the
circumstellar envelope. We find that the C$_2$H$_2$ P and R branches at 7.5
micron are affected by dust dilution at higher mass-loss rates -- albeit to a
lesser extent for sources in the Magellanic Clouds, compared to the Milky Way
-- while the narrow 13.7 micron C$_2$H$_2$ Q branch only shows the effect of
dust dilution at low mass-loss rates. A strong metallicity dependence is not
observed for the Q branch. Independent of metallicity, we also provide an
explanation for the observed shifts in the central wavelength of the SiC
emission feature, as we show that these are largely caused by molecular band
absorption on either side of the dust emission feature, dominating over shifts
in the central wavelength caused by self-absorption. We have devised a method
to study the dust condensation history in carbon-rich AGB stars in different
metallicity environments, by measuring the strength of the 11.3 \um SiC and 30
\um MgS features with respect to the continuum, as a function of mass-loss
rate. With this method, it is possible to distinguish in what order SiC and
graphite condense, which is believed to be sensitive to the metallicity, prior
to the eventual deposit of the MgS layer.
