Arithmetic codes are being increasingly used in the entropy coding stage in many multimedia transmission applications. Combining channel coding with arithmetic coding can give implementation and performance advantages compared to separate source and channel coding. In this work, novel improvements are introduced into a technique by Grangetto et al. that uses maximum a posteriori (MAP) estimation for decoding joint source-channel coding using arithmetic codes. The arithmetic decoder is modified for quicker symbol decoding and error detection by the introduction of a look-ahead technique, and the calculation of the MAP metric is modified for faster error detection. These modifications also result in improved performance compared to the original scheme. Experimental results show an improvement of up to 0.4 dB when using soft-decision decoding and 0.6 dB when using hard-decision decoding.